Control system of compression-ignition engine

ABSTRACT

A control system of a compression-ignition engine is provided, which includes an engine, an injector, a spark plug, and a controller connected to the injector and the spark plug, and configured to operate the engine by outputting a control signal to the injector and the spark plug. After the spark plug ignites mixture gas to start combustion, unburned mixture gas combusts by self-ignition. The controller outputs the control signal to the injector to perform a first-stage injection of fuel and then a second-stage injection in which fuel is injected to at least form the mixture gas around the spark plug. The controller also outputs the control signal to the injector to control a ratio of the injection amount of the second-stage injection with respect to the injection amount of the first-stage injection to be higher at a high engine speed than at a low engine speed.

TECHNICAL FIELD

The present disclosure relates to a control system of acompression-ignition engine.

BACKGROUND OF THE DISCLOSURE

JP4082292B discloses an engine for combusting a mixture gas inside acombustion chamber by self-ignition within a given operating range wherean engine load and an engine speed are low. The engine combusts themixture gas by spark-ignition within an operating range where the engineload is higher than the given operating range and an operating rangewhere the engine speed is higher than the given operating range.

Incidentally, combustion caused by compression ignition accompaniesrelatively loud combustion noise. When the engine speed is high, NVH(Noise Vibration Harshness) of the engine exceeds an allowable value.

SUMMARY OF THE DISCLOSURE

The present disclosure is made in view of the above situations and aimsto perform combustion by compression ignition while suppressing NVH of acompression-ignition engine below an allowable value.

The present inventors considered a combustion mode in which SI (SparkIgnition) combustion and CI (Compression Ignition) combustion (orself-ignition, AI (Auto Ignition) combustion) are combined. The SIcombustion is combustion accompanying flame propagation which starts byforcibly igniting the mixture gas inside a combustion chamber. The CIcombustion is combustion which starts by the mixture gas inside thecombustion chamber igniting by being compressed. In the combustion modecombining the SI combustion and the CI combustion, a spark plug forciblyignites the mixture gas inside the combustion chamber to combust itthrough flame propagation, and heat generated by this combustion raisesthe temperature inside the combustion chamber, which leads to combustionof unburned mixture gas by self-ignition.

In the CI combustion, the timing of the self-ignition changes greatlydue to a variation in the temperature inside the combustion chamberbefore the compression starts. For example, if the timing of theself-ignition advances, the combustion noise increases.

In this regard, the variation in the temperature inside the combustionchamber before the compression starts can be reduced by adjusting theheat generation amount in the SI combustion. For example, by controllingthe ignition timing to adjust the start timing of the SI combustionaccording to the temperature inside the combustion chamber before thecompression starts, the unburned mixture gas can self-ignite at a targettiming. Hereinafter, the combustion mode in which the SI combustion andthe CI combustion are combined so that the CI combustion is controlledusing the SI combustion is referred to as SPCCI (SPark ControlledCompression Ignition) combustion.

The combustion by flame propagation causes a relatively small pressurevariation, thus reducing the combustion noise. Further, the CIcombustion shortens the combustion period compared to the combustion byflame propagation, which is advantageous in improving fuel efficiency.Therefore, the combustion mode combining the SI combustion and the CIcombustion improves fuel efficiency while reducing the combustion noise.

By performing the SPCCI combustion when an engine speed is high, it ispossible to perform the CI combustion while suppressing NVH below theallowable value.

Specifically, according to one aspect of the present disclosure, acontrol system of a compression-ignition engine is provided, whichincludes an engine configured to cause combustion of a mixture gasinside a combustion chamber, an injector attached to the engine andconfigured to inject fuel into the combustion chamber, a spark plugdisposed to be oriented into the combustion chamber and configured toignite the mixture gas inside the combustion chamber, and a controllerconnected to the injector and the spark plug, and configured to operatethe engine by outputting a control signal to the injector and the sparkplug, respectively.

After the spark plug ignites the mixture gas to start combustion,unburned mixture gas combusts by self-ignition. The controller outputsthe control signal to the injector to perform a first-stage injection ofthe fuel and, after the first-stage injection, a second-stage injectionin which the fuel is injected to at least form the mixture gas aroundthe spark plug. The controller also outputs the control signal to theinjector to control a ratio of the injection amount of the second-stageinjection with respect to the injection amount of the first-stageinjection to be higher at a high engine speed than at a low enginespeed.

Note that the definition of “combustion chamber” here is not limited toa space formed when a piston is at a top dead center on compressionstroke (CTDC) but is broad.

According to this configuration, the injector performs the first-stageinjection and the second-stage injection in response to the controlsignal from the controller. The second-stage injection at least formsthe mixture gas around the spark plug. The first-stage injection formsthe mixture gas around the mixture gas to be formed by the second-stageinjection.

The spark plug forcibly ignites the mixture gas in the combustionchamber, particularly, the mixture gas around the spark plug, inresponse to the control signal from the controller. The mixture gasaround the spark plug combusts by flame propagation. After thecombustion starts by the flame propagation, the combustion ends by theunburned mixture gas inside the combustion chamber combusting byself-ignition. In the combustion chamber, the SPCCI combustion isperformed. As described above, the SPCCI combustion achieves bothprevention of the combustion noise and improvement in the fuelefficiency.

The controller increases the injection amount of the second-stageinjection at the high engine speed compared to at the low engine speed.By increasing the injection amount of the second-stage injection, sincea concentration of the mixture gas formed around the spark plugincreases, the SI combustion in the SPCCI combustion becomes sharp. As aresult, the SI combustion is sufficiently performed until the unburnedmixture gas self-ignites, and a ratio of the CI combustion in the SPCCIcombustion decreases. Thus, the combustion noise generated by the SPCCIcombustion is reduced, and when the engine speed is high, NVH issuppressed below the allowable value.

Further, according to another aspect of the present disclosure, acontrol system of a compression-ignition engine is provided, whichincludes an engine configured to cause combustion of a mixture gasinside a combustion chamber, an injector attached to the engine andconfigured to inject fuel into the combustion chamber, a spark plugdisposed adjacent to the injector and configured to ignite the mixturegas inside the combustion chamber, and a controller operativelyconnected to the injector and the spark plug and configured to operatethe engine by outputting a control signal to the injector and the sparkplug, respectively.

After the spark plug ignites the mixture gas to start combustion,unburned mixture gas combusts by self-ignition. The controller outputsthe control signal to the injector to perform a first-stage injection ofthe fuel in a period from intake stroke to an early half of compressionstroke, and a second-stage injection in a period from a latter half ofthe compression stroke to an early half of expansion stroke. Thecontroller also outputs the control signal to the injector to control aratio of the injection amount of the second-stage injection with respectto the injection amount of the first-stage injection to be higher at ahigh engine speed than at a low engine speed.

According to this configuration, similarly to the above description, thecontroller outputs the control signal to the injector to perform thefirst-stage injection of the fuel in the period from the intake stroketo the early half of the compression stroke, and the second-stageinjection in the period from the latter half of the compression stroketo the early half of the expansion stroke. The early half of thecompression stroke may be defined by evenly dividing the compressionstroke into two parts of the early half and the latter half. Similarly,the latter half of the compression stroke may be defined by evenlydividing the compression stroke into two parts of the early half and thelatter half. The early half of the expansion stroke may be defined byevenly dividing the expansion stroke into two parts of the early halfand the latter half. In the second-stage injection, since the injectiontiming is late, the mixture gas is formed near the spark plug adjacentto the injector, whereas in the first-stage injection, since theinjection timing is early, the mixture gas is formed at a positioninside the combustion chamber away from the injector and the spark plug.

The spark plug forcibly ignites the mixture gas near the spark plug inresponse to the control signal from the controller. The mixture gascombusts by flame propagation. After the combustion starts by flamepropagation, the combustion ends by the unburned mixture gas away fromthe spark plug combusting by self-ignition. That is, in the combustionchamber, the SPCCI combustion is performed.

Further, the controller increases the injection amount of thesecond-stage injection at the high engine speed. Thus, since aconcentration of the mixture gas near the spark plug increases, the SIcombustion in the SPCCI combustion becomes sharp and the ratio of the CIcombustion decreases. As a result, the combustion noise is reduced, andtherefore, when the engine speed is high, NVH is suppressed below theallowable value.

The controller may output the control signal to the injector so that theratio of the injection amount of the second-stage injection changes at agiven change rate as the engine speed changes. The controller may causethe change rate to be higher at a high engine speed than at a low enginespeed.

Thus, the ratio of the injection amount of the second-stage injectionincreases at the given change rate as the engine speed increases. Whenthe engine speed is high, the SI combustion in the SPCCI combustionbecomes sharp, which is advantageous in reducing the combustion noise.Therefore, when the engine speed is high, NVH is suppressed below theallowable value.

When the engine speed is equal to or lower than a first given speed, thecontroller may output the control signal to the injector so that theratio of the injection amount of the second-stage injection is constanteven when the engine speed changes. When the engine speed exceeds thefirst given speed, the controller may output the control signal to theinjector so that the ratio of the injection amount of the second-stageinjection increases as the engine speed increases.

Since NVH of the engine is small when the engine speed is low, ratherthan reducing the combustion noise, performing CI combustion at a highratio in the SPCCI combustion is advantageous in improving fuelefficiency. When the engine speed is equal to or lower than the firstgiven speed, the control signal is outputted to the injector so that theratio of the injection amount of the second-stage injection is constanteven when the engine speed changes. This corresponds to the change rateof the ratio of the injection amount of the second-stage injectionchanging as the engine speed changes being zero. By having the injectionamount of the second-stage injection small and the change rate zero whenthe engine speed is low, the CI combustion is sufficiently performed inthe SPCCI combustion, which improves fuel efficiency.

On the other hand, since NVH of the engine becomes larger when theengine speed increases, it is required to reduce the combustion noise.When the engine speed exceeds the first given speed, the control signalis outputted to the injector so that the ratio of the injection amountof the second-stage injection increases as the engine speed increases.This corresponds to the change rate of the ratio of the injection amountof the second-stage injection changing as the engine speed changesexceeding zero. When the engine speed is high, the SI combustion issufficiently performed in the SPCCI combustion, which reduces thecombustion noise.

When the engine speed exceeds a second given speed that is higher thanthe first given speed, the controller may output the control signal tothe injector so that the ratio of the injection amount of thesecond-stage injection becomes a given value.

In the second-stage injection, since the timing for injecting the fuelin terms of a crank angle is late, the period until the injected fuelforms the combustible mixture gas is short. Further, as the engine speedincreases, the time for the crank angle to change by the same angle isshorter. Therefore, the time length from the injection of the fuel inthe second-stage injection to the ignition becomes shorter as the enginespeed increases.

As described above, when the ratio of the injection amount of thesecond-stage injection is increased as the engine speed increases, alarge amount of fuel needs to be vaporized in a short period of time toform the mixture gas. Here, in reality, the amount of fuel which doesnot combust in the SI combustion of the SPCCI combustion increases, alarge amount of fuel causes the CI combustion, and thus the combustionnoise may increase.

Therefore, in a configuration in which the second-stage injection ratiois increased as the engine speed increases, when the engine speedexceeds the second given speed that is higher than the first givenspeed, the control signal is outputted to the injector so that the ratioof the injection amount of the second-stage injection becomes the givenvalue. That is, when the engine speed exceeds the second given speed,the injection amount of the second-stage injection is fixed at a givenamount regardless of the engine speed. Thus, the fuel which is notcombusted in the SI combustion is prevented from increasing, and thecombustion noise is avoided from increasing due to the CI combustion.

The control system may further include an intake flow control deviceattached to the engine and configured to adjust a flow of intake airintroduced into the combustion chamber. When the engine speed exceedsthe second given speed, the controller may output the control signal tothe intake flow control device to strengthen the flow of the intake air.

If the injection amount of the second-stage injection is fixed at thegiven amount when the engine speed exceeds the second given speed asdescribed above, the effect of reducing the combustion noise caused byincreasing the injection amount of the second-stage injection islimited.

Therefore, with the above configuration, when the engine speed exceedsthe second given speed, the flow of the intake air is strengthened bythe intake flow control device. Thus, a vaporization of the fuelinjected by the second-stage injection is stimulated and the SIcombustion is performed with the strong flow inside the combustionchamber, which causes the SI combustion in the SPCCI combustion to besharp. Therefore, even when the engine speed exceeds the second givenspeed, the combustion noise is reduced.

The controller may output the control signal to the intake flow controldevice to strengthen the flow of the intake air as the engine speedincreases.

Thus, when the engine speed is high, since the SI combustion becomessharp by the strong intake flow, the combustion noise is reduced. Theoperating range in which the combustion is performed by the compressionignition is further extended to the higher engine speed side.

The controller may output the control signal to the injector so that aninjection timing of the second-stage injection is advanced when theengine operates at a high speed compared to when the engine operates ata low speed.

When the engine speed increases, the vaporization period from theinjection of the fuel into the combustion chamber to the ignitionthereof becomes shorter. In the SPCCI combustion, the amount of themixture gas which combusts in the SI combustion decreases and the CIcombustion increases. Since the ratio of the injection amount of thesecond-stage injection is increased when the engine speed is high, theratio of the CI combustion increases by the decreased amount of themixture gas which combusts in the SI combustion. As a result, thecombustion noise of the SPCCI combustion increases and NVH may exceedthe allowable value.

By advancing the injection timing, the vaporization period is extendedand the amount of the mixture gas which combusts in the SI combustionincreases. Since the combustion noise of the SPCCI combustion isreduced, when the engine speed is high, NVH is suppressed below theallowable value.

The engine may include a piston constituting the combustion chamber, thepiston being formed with a cavity facing the injector, by indenting anupper surface of the piston. In the first-stage injection, the fuel maybe injected into a squish area outside the cavity on compression stroke,and in the second-stage injection, the fuel may be injected into thecavity.

According to this configuration, the mixture gas in the cavity combustsin the SI combustion. Here, the phrase “the section within the cavity”may mean a combination of a section from a projection plane of anopening surface of the cavity on the ceiling surface of the combustionchamber to the opening surface of the cavity and a section inside thecavity. By injecting the fuel toward the cavity, homogeneous mixture gasis formed inside the cavity and a flow of gas inside the section withinthe cavity is strengthened. Thus, the spark plug ignites the mixture gasunder a state where turbulence kinetic energy inside the section withinthe cavity is high. Therefore, since the SI combustion becomes sharp,the combustion noise is reduced also at the high engine speed.

The controller may set an SI ratio to be lower than 100% and set the SIratio to be higher at a high engine speed than at a low engine speed,the SI ratio being an index relating to a ratio of a heat amountgenerated when the ignited mixture gas combusts by flame propagationwith respect to a total heat amount generated when the mixture gasinside the combustion chamber combusts.

In the combustion mode in which the mixture gas combusts by the flamepropagation and, after the combustion starts by the flame propagation,the combustion ends by the unburned mixture gas combusting by theself-ignition (i.e., SPCCI combustion), the SI ratio is set to be lowerthan 100%. In the combustion mode in which the combustion ends only withthe combustion by the flame propagation without the combustion by theself-ignition, the SI ratio is set to 100%

When the SI ratio is increased in the SPCCI combustion, the ratio of theSI combustion increases, which is advantageous in reducing thecombustion noise. On the other hand, when the SI ratio is decreased inthe SPCCI combustion, the ratio of the CI combustion increases, which isadvantageous in improving fuel efficiency.

In the above configuration, the ratio of the injection amount of thesecond-stage injection is increased at the high engine speed compared toat the low engine speed. Thus, the SI ratio in the SPCCI combustion isincreased. Therefore, the combustion noise is reduced and, even when theengine speed is high, the SPCCI combustion is performed.

The control system may further include a swirl generating partconfigured to generate a swirl flow inside the combustion chamber. Thecontroller may output the control signal to the swirl generating part togenerate the swirl flow inside the combustion chamber regardless of theengine speed.

If the swirl generating part generates the swirl flow inside thecombustion chamber, the SI combustion in the SPCCI combustion becomessharp. By generating the swirl flow inside the combustion chamberregardless of the engine speed, the combustion noise caused by the SPCCIcombustion is reduced without increasing the injection amount of thesecond-stage injection. Since the vaporization time for the fuelinjected by the second-stage injection is short, unburned fuel or sootmay be generated. Thus, by not increasing the injection amount of thesecond-stage injection, the generation of unburned mixture gas or sootis reduced. That is, generating the swirl flow inside the combustionchamber is advantageous in improving an exhaust emission performance ofthe engine.

At least within a highest speed segment of an operating range of theengine in which the spark plug ignites the mixture gas to start thecombustion and then the unburned mixture gas combusts by self-ignition,the controller may output the control signal to the injector so that theratio of the injection amount of the second-stage injection becomeshigher at a high engine speed than at a low engine speed.

As described above, by generating the swirl flow inside the combustionchamber, the injection amount of the second-stage injection is reduced.Therefore, in a configuration in which the injection ratio of thesecond-stage injection is increased when the engine speed is high, whenthe engine speed is in a highest speed range in which the SPCCIcombustion is performed, the injection amount of the second-stageinjection is increased without any limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration of an engine.

FIG. 2 is a diagram illustrating a structure of a combustion chamber, inwhich the upper part is a plan view of the combustion chamber and thelower part is a II-II cross-sectional view.

FIG. 3 is a plan view illustrating structures of the combustion chamberand an intake system.

FIG. 4 is a block diagram illustrating a configuration of a controldevice of the engine.

FIG. 5 is a diagram illustrating a rig test device for measuring a swirlratio.

FIG. 6 is a chart illustrating a relationship between an opening ratioof a secondary passage and the swirl ratio.

FIG. 7A is a chart illustrating an operating range map of the engine.

FIG. 7B is a chart illustrating another operating range map of theengine.

FIG. 7C is a chart illustrating another operating range map of theengine.

FIG. 8 shows charts in which the upper part conceptually illustrates achange in heat generation rate in SPCCI combustion in which SIcombustion and CI combustion are combined, the middle part illustrates adefinition of an SI ratio in the SPCCI combustion, and the lower partillustrates another definition of the SI ratio in the SPCCI combustion.

FIG. 9 is a chart illustrating a change in the SI ratio, a change in astate function inside a combustion chamber, a change in an overlapperiod between an intake valve and an exhaust valve, and changes in aninjection timing and ignition timing of fuel, with respect to an engineload.

FIG. 10 shows charts in which the upper part illustrates a change in acombustion waveform due to an increase in the engine load in non-boostedSPCCI combustion, and the lower part illustrates a change in acombustion waveform due to an increase in the engine load in boostedSPCCI combustion.

FIG. 11 shows charts in which the upper part illustrates one example ofa relationship between an engine speed and a second-stage injectionratio within an operating range in which the SPCCI combustion isperformed, and the lower part illustrates another example of therelationship between the engine speed and the second-stage injectionratio within the operating range in which the SPCCI combustion isperformed.

FIG. 12 shows charts in which the upper part illustrates one example ofa relationship between the engine speed and a second-stage injectiontiming within the operating range in which the SPCCI combustion isperformed, and the lower part illustrates another example of therelationship between the engine speed and the second-stage injectiontiming within the operating range in which the SPCCI combustion isperformed.

FIG. 13 shows charts in which the upper part illustrates one example ofa relationship between the engine speed and an opening of a swirlcontrol valve within the operating range on the operating range map ofFIG. 7A in which the SPCCI combustion is performed, and the lower partillustrates another example of the relationship between the engine speedand the opening of the swirl control valve within the operating range onthe operating range map of FIG. 7C in which the SPCCI combustion isperformed.

FIG. 14 is a chart illustrating one example of a relationship betweenthe engine speed and the SI ratio within the operating range in whichthe SPCCI combustion is performed.

FIG. 15 is a flowchart illustrating a flow of a control of the engineexecuted by an ECU.

FIG. 16 is a diagram illustrating a control concept regarding anadjustment of the SI ratio.

FIG. 17 is a diagram illustrating fuel injection timings, ignitiontimings, and combustion waveforms in respective operating states on theoperating range map of FIG. 7C.

FIG. 18 shows charts in which the upper part illustrates one example ofa relationship between the engine speed and the second-stage injectionratio on the operating range map of FIG. 7C, and the lower partillustrates another example of the relationship between the engine speedand the second-stage injection ratio.

FIG. 19 shows charts in which the upper part illustrates one example ofa relationship between the engine speed and the second-stage injectiontiming on the operating range map of FIG. 7C, and the lower partillustrates another example of the relationship between the engine speedand the second-stage injection timing.

FIG. 20 is a chart illustrating another example of the operating rangeof the engine.

FIG. 21 is a diagram illustrating combustion waveforms in respectiveoperating states illustrated in FIG. 20.

DETAILED DESCRIPTION OF THE DISCLOSURE

Hereinafter, embodiments of a control system of a compression-ignitionengine are described in detail with reference to the accompanyingdrawings. The following description gives one example of the controlsystem of the compression-ignition engine. FIG. 1 is a diagramillustrating a configuration of the compression-ignition engine. FIG. 2is a cross-sectional view illustrating a structure of a combustionchamber, in which the upper part is a plan view of the combustionchamber and the lower part is a II-II cross-sectional view. FIG. 3 is aplan view illustrating structures of the combustion chamber and anintake system. Note that in FIG. 1, an intake side is on the left sideand an exhaust side is on the right side of the drawing sheet. Furtherin FIGS. 2 and 3, the intake side is on the right side and the exhaustside is on the left side of the drawing sheets. FIG. 4 is a blockdiagram illustrating a configuration of a control device of thecompression-ignition engine.

An engine 1 is a four-stroke engine which is operated by a combustionchamber 17 repeating intake stroke, compression stroke, expansionstroke, and exhaust stroke. The engine 1 is mounted on a four-wheelautomobile. The automobile travels by the operation of the engine 1.Fuel of the engine 1 is gasoline in this embodiment. The gasoline maycontain bioethanol, etc. The fuel of the engine 1 may be any kind offuel as long as it is a liquid fuel containing at least gasoline.

(Engine Configuration)

The engine 1 includes a cylinder block 12 and a cylinder head 13 placedon the cylinder block 12. The cylinder block 12 is formed therein with aplurality of cylinders 11. In FIGS. 1 and 2, only one cylinder 11 isillustrated. The engine 1 is a multi-cylinder engine.

A piston 3 is reciprocatably inserted in each cylinder 11. The piston 3is coupled to a crankshaft 15 via a connecting rod 14. The piston 3defines the combustion chamber 17 together with the cylinder 11 and thecylinder head 13. Note that the definition of “combustion chamber” isnot limited to a space formed when the piston 3 is at a top dead centeron compression stroke (CTDC) but may be broad. That is, “combustionchamber” may mean any space formed by the piston 3, the cylinder 11 andthe cylinder head 13 regardless of the position of the piston 3.

As illustrated in FIG. 2, a lower surface of the cylinder head 13, thatis, a ceiling surface of the combustion chamber 17, is formed by aninclined surface 1311 and an inclined surface 1312. The inclined surface1311 inclines upwardly toward an injection axis X2 (an axis passingthrough the center of injection of an injector 6 described later) fromthe intake side. The inclined surface 1312 inclines upwardly toward theinjection axis X2 from the exhaust side. The ceiling surface of thecombustion chamber 17 has a so-called pent-roof shape.

An upper surface of the piston 3 bulges toward the ceiling surface ofthe combustion chamber 17. A cavity 31 is formed in the upper surface ofthe piston 3. The cavity 31 is formed by indenting the upper surface ofthe piston 3. The cavity 31 has a shallow plate shape. The cavity 31faces the injector 6 when the piston 3 is located near CTDC.

The center of the cavity 31 is offset from a center axis X1 of thecylinder 11 to the exhaust side and coincides with the injection axis X2of the injector 6. The cavity 31 has a convex section 311. The convexsection 311 is formed on the injection axis X2 of the injector 6. Theconvex section 311 has a substantially conical shape. The convex section311 extends upwardly toward the ceiling surface of the cylinder 11 fromthe bottom of the cavity 31.

The cavity 31 has an indented section 312 formed to surround the convexsection 311 entirely. The cavity 31 has a symmetric shape with respectto the injection axis X2.

A circumferential side face of the indented section 312 extends from abottom surface of the cavity 31 to an opening surface of the cavity 31,inclined with respect to the injection axis X2. An inner diameter of thecavity 31 at the indented section 312 gradually increases from thebottom surface of the cavity 31 to the opening surface of the cavity 31.

Note that the shape of the combustion chamber 17 is not limited to thatillustrated in FIG. 2. The shapes of the cavity 31, the upper surface ofthe piston 3, the ceiling surface of the combustion chamber 17, etc. aresuitably changeable. For example, the depth of the indented section 312may be shallower on the outer circumferential side. In this case, anamount of EGR (Exhaust Gas Recirculation) gas around a spark plug 25(described later) decreases, and flame propagation of SI combustion inSPCCI combustion (also described later) becomes favorable. Furthermore,the cavity 31 may not have the convex section 311.

The geometric compression ratio of the engine 1 is set high in order toimprove a theoretical thermal efficiency and stabilize CI combustiondescribed later. For example, the geometric compression ratio of theengine 1 is 14:1 or higher. The geometric compression ratio may be 18:1,for example. The geometric compression ratio may suitably be set withina range of 14:1 to 20:1.

The cylinder head 13 is formed with an intake port 18 for each cylinder11. As illustrated in FIG. 3, the intake port 18 includes two intakeports of a first intake port 181 and a second intake port 182. The firstintake port 181 and the second intake port 182 are arranged in axialdirections of the crankshaft 15, i.e., front-and-rear directions of theengine 1. The intake port 18 communicates with the combustion chamber17. Although not illustrated in detail, the intake port 18 is aso-called tumble port. That is, the intake port 18 has such a shape thata tumble flow is formed in the combustion chamber 17.

An intake valve 21 is disposed in the intake port 18. The intake valve21 opens and closes the intake port 18 to and from the combustionchamber 17. The intake valve 21 is opened and closed by a valveoperating mechanism at a given timing. This valve operating mechanismmay be a variable valve mechanism which makes a valve timing and/orvalve lift variable. In this configuration example, as illustrated inFIG. 4, the variable valve mechanism has an intake electrically-operatedS-VT (Sequential-Valve Timing) 23. The intake electrically-operated S-VT23 is continuously variable of a rotational phase of an intake camshaftwithin a given angular range. Thus, the open and close timings of theintake valve 21 continuously change. Note that the intake valveoperating mechanism may have a hydraulically-operated S-VT instead ofthe electrically-operated S-VT.

The cylinder head 13 is also formed with an exhaust port 19 for eachcylinder 11. As illustrated in FIG. 3, the exhaust port 19 also includestwo exhaust ports of a first exhaust port 191 and a second exhaust port192. The first exhaust port 191 and the second exhaust port 192 arearranged in the front-and-rear directions of the engine 1. The exhaustport 19 communicates with the combustion chamber 17.

An exhaust valve 22 is disposed in the exhaust port 19. The exhaustvalve 22 opens and closes the exhaust port 19 to and from the combustionchamber 17. The exhaust valve 22 is opened and closed by a valveoperating mechanism at a given timing. This valve operating mechanismmay be a variable valve mechanism which makes a valve timing and/orvalve lift variable. In this configuration example, as illustrated inFIG. 4, the variable valve mechanism has an exhaustelectrically-operated S-VT 24. The exhaust electrically-operated S-VT 24is continuously variable of a rotational phase of an exhaust camshaftwithin a given angular range. Thus, the open and close timings of theexhaust valve 22 continuously change. Note that the exhaust valveoperating mechanism may have a hydraulically-operated S-VT instead ofthe electrically-operated S-VT.

Although is described later in detail, the engine 1 adjusts the lengthof an overlap period of the open timing of the intake valve 21 and theclose timing of the exhaust valve 22 by the intake electrically-operatedS-VT 23 and the exhaust electrically-operated S-VT 24. Thus, residualgas in the combustion chamber 17 is scavenged. Further, by adjusting thelength of the overlap period, internal EGR gas is introduced into thecombustion chamber 17 or is confined in the combustion chamber 17. Inthis configuration example, the intake electrically-operated S-VT 23 andthe exhaust electrically-operated S-VT 24 constitute an internal EGRsystem as one of state function setting devices. Note that the internalEGR system is not necessarily constituted by the S-VT.

The injector 6 is attached to the cylinder head 13 for each cylinder 11.The injector 6 injects the fuel directly into the combustion chamber 17.The injector 6 is disposed in a valley portion of the pent roof wherethe inclined surface 1311 on the intake side and the inclined surface1312 on the exhaust side intersect. As illustrated in FIG. 2, theinjector 6 is disposed so that its injection axis X2 is located on theexhaust side of the center axis X1. The injection axis X2 of theinjector 6 is parallel to the center axis X1. As described above, theinjection axis X2 of the injector 6 coincides with the position of theconvex section 311 of the cavity 31. The injector 6 is oriented towardthe cavity 31. Note that the injection axis X2 of the injector 6 maycoincide with the center axis X1 of the cylinder 11. Also in this case,it is desirable that the injection axis X2 of the injector 6 coincidewith the position of the convex section 311 of the cavity 31.

Although not illustrated in detail, the injector 6 is constructed by amulti-port fuel injector having a plurality of nozzle ports. Asindicated by two-dotted chain lines in FIG. 2, the injector 6 injectsthe fuel so that the fuel spray radially spreads from the radial centerof the combustion chamber 17. In this configuration example, theinjector 6 has ten nozzle ports, and the nozzle ports are arranged at aneven angular interval in the circumferential direction. As illustratedin the upper part of FIG. 2, the axes of the nozzle ports do notcircumferentially overlap with the spark plug 25 described later. Thatis, the spark plug 25 is sandwiched between the axes of two adjacentnozzle ports. Thus, the fuel spray injected from the injector 6 isprevented from directly hitting the spark plug 25 and getting anelectrode wet.

As described later, the injector 6 may inject the fuel at the timingwhen the piston 3 is positioned near CTDC. In this case, when theinjector 6 injects the fuel, the fuel spray flows downward along theconvex section 311 of the cavity 31 while mixing with fresh air, andflows along the bottom surface and the circumferential surface of theconcave portion 312 to spread radially outward from the center of thecombustion chamber 17. Then, the mixture gas reaches the opening of thecavity 31, flows along the inclined surface 1311 on the intake side andthe inclined surface 1312 on the exhaust side, and further flows fromthe outer circumferential side toward the center of the combustionchamber 17.

Note that the injector 6 is not limited to the multi-port injector. Theinjector 6 may adopt an outward-opening valve injector.

A fuel supply system 61 is connected to the injector 6. The fuel supplysystem 61 includes a fuel tank 63 configured to store the fuel and afuel supply path 62 connecting the fuel tank 63 with the injector 6. Afuel pump 65 and a common rail 64 are provided in the fuel supply path62. The fuel pump 65 pumps the fuel to the common rail 64. In thisembodiment, the fuel pump 65 is a plunger pump which is driven by thecrankshaft 15. The common rail 64 stores the fuel pumped from the fuelpump 65 at high fuel pressure. When the injector 6 opens, the fuelstored in the common rail 64 is injected into the combustion chamber 17from the nozzle ports of the injector 6. The fuel supply system 61 isable to supply the fuel at a high pressure of 30 MPa or higher to theinjector 6. A highest fuel pressure of the fuel supply system 61 may be,for example, about 120 MPa. The pressure of the fuel supplied to theinjector 6 may be changed according to an operating state of the engine1. Note that the structure of the fuel supply system 61 is not limitedto the above structure.

The spark plug 25 is attached to the cylinder head 13 for each cylinder11. The spark plug 25 forcibly ignites the mixture gas in the combustionchamber 17. In this configuration example, the spark plug 25 is disposedat the intake side of the cylinder 11 with respect to the center axisX1. The spark plug 25 is located between the two intake ports 18. Thespark plug 25 is attached to the cylinder head 13 to extend downwardly,toward the center of the combustion chamber 17 in a tilted posture withrespect to up-and-down directions of the cylinder head 13. Asillustrated in FIG. 2, the electrode of the spark plug 25 is locatednear the ceiling surface of the combustion chamber 17 to be orientedtoward inside the combustion chamber 17. Note that the disposed positionof the spark plug 25 is not limited to the configuration example of FIG.2. The spark plug 25 may be disposed on the exhaust side of the centeraxis X1 of the cylinder 11. Alternatively, the spark plug 25 may bedisposed on the center axis X1 of the cylinder 11, and the injector 6may be disposed on the intake side or the exhaust side with respect tothe center axis X1.

An intake passage 40 is connected to one side of the engine 1. Theintake passage 40 communicates with the intake ports 18 of the cylinders11. The intake passage 40 is a passage through which gas to beintroduced into the combustion chamber 17 flows. An air cleaner 41 whichfilters fresh air is disposed in an upstream end part of the intakepassage 40. A surge tank 42 is disposed near a downstream end of theintake passage 40. Although not illustrated in detail, a part of theintake passage 40 downstream of the surge tank 42 constitutesindependent passages branched for the respective cylinders 11.Downstream ends of the independent passages communicate with the intakeports 18 of the cylinders 11, respectively.

A throttle valve 43 is disposed in the intake passage 40 between the aircleaner 41 and the surge tank 42. The throttle valve 43 adjusts anintroduction amount of fresh air into the combustion chamber 17 byadjusting an opening thereof. The throttle valve 43 constitutes one ofthe state function setting devices.

A booster 44 is disposed in the intake passage 40 downstream of thethrottle valve 43. The booster 44 boosts the gas introduced into thecombustion chamber 17. In this configuration example, the booster 44 isa supercharger which is driven by the engine 1. The booster 44 may be,for example, of a Roots type. The booster 44 may have any structure. Thebooster 44 may be of a Lisholm type, a Vane type, or a centrifugal type.Note that the booster may be an electric booster or a turbocharger whichis driven by exhaust energy.

An electromagnetic clutch 45 is interposed between the booster 44 andthe engine 1. The electromagnetic clutch 45 controls the flow of adriving force between the booster 44 and the engine 1, for example, ittransmits the driving force from the engine 1 to the booster 44 orinterrupts the transmission of the driving force therebetween. As isdescribed later, by an ECU 10 switching the connection/disconnection ofthe electromagnetic clutch 45, the on/off of the booster 44 is switched.That is, in this engine 1, boosting the gas to be introduced into thecombustion chamber 17 by the booster 44 and not boosting the gas to beintroduced into the combustion chamber 17 by the booster 44 areswitchable therebetween.

An intercooler 46 is disposed in the intake passage 40 downstream of thebooster 44. The intercooler 46 cools the gas compressed in the booster44. The intercooler 46 may be, for example, of a water cooling type.

A bypass passage 47 is connected to the intake passage 40. The bypasspassage 47 connects a part of intake passage 40 upstream of the booster44 to a part of the intake passage 40 downstream of the intercooler 46so as to bypass the booster 44 and the intercooler 46. For example, thebypass passage 47 is connected to the surge tank 42. An air bypass valve48 is disposed in the bypass passage 47. The air bypass valve 48 adjustsa flow rate of the gas flowing through the bypass passage 47.

When the booster 44 is turned off (that is, when the electromagneticclutch 45 is disconnected), the air bypass valve 48 is fully opened.Thus, the gas flowing through the intake passage 40 bypasses the booster44 and is introduced into the combustion chamber 17 of the engine 1. Theengine 1 operates in a non-boosted state, that is, in a naturallyaspirated state.

When the booster 44 is turned on (that is, when the electromagneticclutch 45 is connected), the gas passed through the booster 44 partiallyflows back upstream of the booster 44 through the bypass passage 47. Bycontrolling an opening of the air bypass valve 48, a backflow amount isadjusted, which leads to adjusting boosting pressure of the gasintroduced into the combustion chamber 17. Note that the term “boosted”may be defined as when the pressure in the surge tank 42 exceedsatmospheric pressure, and the term “non-boosted” may be defined as whenthe pressure in the surge tank 42 falls below the atmospheric pressure.In this configuration example, a boosting system 49 is comprised of thebooster 44, the bypass passage 47, and the air bypass valve 48. The airbypass valve 48 constitutes one of the state function setting devices.

The engine 1 has a swirl generating part which generates a swirl flow inthe combustion chamber 17. The swirl generating part is one example ofan intake flow control device. In this configuration example, asillustrated in FIG. 3, the swirl generating part is a swirl controlvalve (SCV) 56 attached to the intake passage 40. The SCV 56 is disposedin a passage. The passage is one of a primary passage 401 and asecondary passage 402 communicating with the first intake port 181 andthe second intake port 182, respectively. The SCV 56 is an openingregulating valve which is capable of adjusting the opening of a crosssection of the secondary passage. When the opening of the SCV 56 issmall, the flow rate of the intake air into the combustion chamber 17from the first intake port 181 relatively increases while the flow rateof the intake air into the combustion chamber 17 from the second intakeport 182 is relatively reduced. Thus, the swirl flow in the combustionchamber 17 becomes strong. When the opening of the SCV 56 is large, theflow rates of the intake air into the combustion chamber 17 from thefirst intake port 181 and the second intake port 182 becomesubstantially even, and thus the swirl flow in the combustion chamber 17becomes weak. When the SCV 56 is fully opened, a swirl flow does notoccur. Note that the swirl flow circulates in the counter-clockwisedirection in FIG. 3 as indicated by the white outlined arrows (also seethe white outlined arrows in FIG. 2).

Note that alternatively/additionally to attaching the SCV 56 to theintake passage 40, the swirl generating part may adopt a structure inwhich the open periods of the two intake valves 21 are varied so as tointroduce the intake air into the combustion chamber 17 from only one ofthe intake valves 21. By opening only one of the two intake valves 21,the intake air is introduced unevenly into the combustion chamber 17,and thus, the swirl flow is generated in the combustion chamber 17.Alternatively, the shapes of the intake ports 18 may be devised so thatthe swirl generating part generates the swirl flow in the combustionchamber 17.

Here, the strength of the swirl flow in the combustion chamber 17 isdefined. In this configuration example, the strength of the swirl flowin the combustion chamber 17 is expressed by a “swirl ratio.” The “swirlratio” may be defined as a value obtained by dividing a value which isobtained from measuring an intake flow lateral angular speed for eachvalve lift and integrating the value, by an engine angular speed. Theintake flow lateral angular speed may be obtained based on a measurementusing a rig test device illustrated in FIG. 5. Specifically, the deviceillustrated in FIG. 5 is structured by placing the cylinder head 13upside down on a pedestal to connect the intake port 18 to an intake airsupply device (not illustrated), and placing a cylinder 36 on thecylinder head 13 to connect, at its upper end, to an impulse meter 38having a honeycomb rotor 37. A lower surface of the impulse meter 38 ispositioned at a position 1.75 D (wherein “D” is a cylinder borediameter) away from a mating surface between the cylinder head 13 andthe cylinder 36. The impulse meter 38 measures torque which acts on thehoneycomb rotor 37 by a swirl generated in the cylinder 36 according tothe supply of the intake air (see the arrow in FIG. 5), and the intakeflow lateral angular speed is obtained based on the torque.

FIG. 6 illustrates a relationship between the opening of the SCV 56 ofthe engine 1 and the swirl ratio. In FIG. 6, the opening of the SCV 56is expressed by an opening ratio with respect to the cross section ofthe secondary passage 402 when fully opened. The opening ratio of thesecondary passage 402 is 0% when the SCV 56 is fully closed, andincreases from 0% as the opening of the SCV 56 increases. The openingratio of the secondary passage 402 is 100% when the SCV 56 is fullyopened. As illustrated in FIG. 6, in the engine 1, the swirl ratiobecomes around 6 when the SCV 56 is fully closed. In order to set theswirl ratio to be 4 or higher, the opening of the SCV 56 may be adjustedwithin a range of the opening ratio of 0 to 15%. Moreover, in order toset the swirl ratio to be about 1.5 to 3, the opening of the SCV 56 maybe adjusted within a range of the opening ratio of about 25 to 40%.

An exhaust passage 50 is connected to a side of the engine 1 oppositefrom the intake passage 40. The exhaust passage 50 communicates with theexhaust ports 19 of the cylinders 11. The exhaust passage 50 is apassage through which the exhaust gas discharged from the combustionchamber 17 flows. Although is not illustrated in detail, an upstreampart of the exhaust passage 50 constitutes independent passages branchedfor the respective cylinders 11. Upstream ends of the independentpassages are connected to the exhaust ports 19 of the cylinders 11,respectively.

An exhaust gas purification system having one or more catalyticconverters is disposed in the exhaust passage 50. The exhaust gaspurification system of this configuration example has two catalyticconverters. The catalytic converter on the upstream side is disposed inan engine bay, and has a three-way catalyst 511 and a GPF (GasolineParticulate Filter) 512. The catalytic converter on the downstream sideis disposed outside the engine room and has a three-way catalyst 513.Note that the exhaust gas purification system is not limited to have theillustrated structure. For example, the GPF 512 may be omitted.Moreover, the catalytic converter is not limited to have the three-waycatalyst. Furthermore, the order of arrangements of the three-waycatalyst and the GPF may suitably be changed.

An EGR passage 52 constituting an external EGR system is connectedbetween the intake passage 40 and the exhaust passage 50. The EGRpassage 52 is a passage for recirculating a portion of the burned gas tothe intake passage 40. An upstream end of the EGR passage 52 isconnected to the exhaust passage 50 between the upstream catalyticconverter and the downstream catalytic converter. A downstream end ofthe EGR passage 52 is connected to the intake passage 40 upstream of thebooster 44. For example, the downstream end of the EGR passage 52 isconnected to an intermediate position of the bypass passage 47. The EGRgas flowing through the EGR passage 52 enters the intake passage 40upstream of the booster 44 without passing through the air bypass valve48 of the bypass passage 47.

A water-cooling type EGR cooler 53 is disposed in the EGR passage 52.The EGR cooler 53 cools the burned gas. An EGR valve 54 is also disposedin the EGR passage 52. The EGR valve 54 adjusts the flow rate of theburned gas in the EGR passage 52. By adjusting an opening of the EGRvalve 54, the recirculation amount of the cooled burned gas (i.e.,external EGR gas) is adjusted.

In this configuration example, an EGR system 55 includes an external EGRsystem including the EGR passage 52 and the EGR valve 54, and aninternal EGR system including the intake electrically-operated S-VT 23and the exhaust electrically-operated S-VT 24 described above. The EGRvalve 54 constitutes one of the state function setting devices. In theexternal EGR system, since the EGR passage 52 is connected downstream ofthe upstream catalytic converter and the EGR cooler 53 is provided, theburned gas at a temperature lower than in the internal EGR system issupplied to the combustion chamber 17.

A control system 20 of the compression-ignition engine includes an ECU(Engine Control Unit) 10 configured to operate the engine 1. The ECU 10is a controller based on a well-known microcomputer. The ECU 10 includesa central processing unit (CPU) 101 (i.e., a processor) configured toexecute program(s)/instructions, memory 102 comprised of RAM(s) (RandomAccess Memory) and ROM(s) (Read Only Memory) and configured to store theprogram(s)/instructions and data, and an input/output bus 103 configuredto input and output electric signals. The ECU 10 is one example of a“controller” or “control device.”

As illustrated in FIGS. 1 and 4, various sensors SW1 to SW16 areconnected to the ECU 10. The sensors SW1 to SW16 output detectionsignals to the ECU 10. The sensors include the following sensors.

That is, the sensors include an airflow sensor SW1 disposed in theintake passage 40 downstream of the air cleaner 41 and configured todetect the flow rate of fresh air in the intake passage 40, a firstintake air temperature sensor SW2 also disposed in the intake passage 40downstream of the air cleaner 41 and configured to detect a temperatureof the fresh air, a first pressure sensor SW3 disposed in the intakepassage 40 downstream of the connecting position with the EGR passage 52and upstream of the booster 44, and configured to detect pressure of thegas flowing into the booster 44, a second intake air temperature sensorSW4 disposed in the intake passage 40 downstream of the booster 44 andupstream of the connecting position of the bypass passage 47 andconfigured to detect a temperature of the gas flowed out of the booster44, a second pressure sensor SW5 attached to the surge tank 42 andconfigured to detect pressure of the gas at a position downstream of thebooster 44, pressure sensors SW6 attached to the cylinder head 13corresponding to the cylinders 11 and configured to detect pressure inthe combustion chambers 17, respectively, an exhaust temperature sensorSW7 disposed in the exhaust passage 50 and configured to detect atemperature of the exhaust gas discharged from the combustion chamber17, a linear O₂ sensor SW8 disposed in the exhaust passage 50 upstreamof the upstream catalytic converter and configured to detect an oxygenconcentration within the exhaust gas, a lambda O₂ sensor SW9 disposed inthe upstream catalytic converter downstream of the catalytic converter511 and configured to detect an oxygen concentration within the exhaustgas, a water temperature sensor SW10 attached to the engine 1 andconfigured to detect a temperature of the cooling water, a crank anglesensor SW11 attached to the engine 1 and configured to detect arotational angle of the crankshaft 15, an accelerator opening sensorSW12 attached to an accelerator pedal mechanism and configured to detectan accelerator opening corresponding to an operation amount of anaccelerator pedal, an intake cam angle sensor SW13 attached to theengine 1 and configured to detect a rotational angle of the intakecamshaft, an exhaust cam angle sensor SW14 attached to the engine 1 andconfigured to detect a rotational angle of the exhaust camshaft, an EGRpressure difference sensor SW15 disposed in the EGR passage 52 andconfigured to detect a difference in pressure between positions upstreamand downstream of the EGR valve 54, and a fuel pressure sensor SW16attached to the common rail 64 of the fuel supply system 61 andconfigured to detect pressure of the fuel to be supplied to the injector6.

Based on these detection signals, the ECU 10 determines the operatingstate of the engine 1 and calculates control amounts of the variousdevices. The ECU 10 outputs control signals related to the calculatedcontrol amounts to the injector 6, the spark plug 25, the intakeelectrically-operated S-VT 23, the exhaust electrically-operated S-VT24, the fuel supply system 61, the throttle valve 43, the EGR valve 54,the electromagnetic clutch 45 of the booster 44, the air bypass valve48, and the SCV 56.

For example, the ECU 10 sets a target torque of the engine 1 anddetermines a target boosting pressure, based on the detection signal ofthe accelerator opening sensor SW12 and a preset map. Then, the ECU 10executes a feedback control to bring the boosting pressure to the targetboosting pressure by adjusting the opening of the air bypass valve 48based on the target boosting pressure and a pressure difference betweenthe upstream and downstream sides of the booster 44 obtained from thedetection signals of the first pressure sensor SW3 and the secondpressure sensor SW5.

Further, the ECU 10 sets a target EGR ratio (that is, a ratio of the EGRgas with respect to all the gas in the combustion chamber 17) based onthe operating state of the engine 1 and a preset map. Then, the ECU 10determines a target EGR gas amount based on the target EGR ratio and theintake air amount based on the detection signal of the acceleratoropening sensor SW12. Then, the ECU 10 executes a feedback control tobring the external EGR gas amount introduced into the combustion chamber17 to the target EGR gas amount by adjusting the opening of the EGRvalve 54 based on the pressure difference between the upstream anddownstream sides of the EGR valve 54 obtained from the detection signalof the EGR difference sensor SW15.

Further, the ECU 10 executes an air-fuel ratio feedback control when agiven control condition is satisfied. For example, based on the oxygenconcentrations within the exhaust gas detected by the linear O₂ sensorSW8 and the lambda O₂ sensor SW9, the ECU 10 adjusts the fuel injectionamount of the injector 6 to bring the air-fuel ratio of the mixture gasto a desired value.

Note that details of other controls of the engine 1 by the ECU 10 aredescribed later.

(First Configurational Example of Operating Range Map of Engine)

FIG. 7A illustrates a first configuration example of an operating rangemap of the engine 1. An operating range map 700 of the engine 1 isdetermined by an engine load and an engine speed. The operating rangemap 700 is roughly divided into four ranges based on the engine load andthe engine speed. For example, the four ranges include a low load range(A) including an idle operation, a high load range (C) including a fullengine load, a medium load range (B) between the low load range (A) andthe high load range (C), and a high speed range (D) where the enginespeed is higher than in the low load range (A), the medium load range(B), and the high load range (C). Within the high speed range (D), theengine 1 injects the fuel into the combustion chamber 17 on the intakestroke and performs the SI combustion by spark-ignition.

Further, the engine 1 performs combustion by compression self-ignitionwithin the medium load range (B) in order to improve the fuel efficiencyand exhaust gas performance. Hereinafter, the combustion modes in eachof the low load range (A), the medium load range (B), and the high loadrange (C) will be described in detail.

(Low Load Range)

The combustion mode when the operating state of the engine 1 is withinthe low load range (A) is the SI combustion in which the spark plug 25ignites the mixture gas inside the combustion chamber 17 to combust itby flame propagation. This is for prioritizing reliably securingcombustion stability. Hereinafter, the combustion mode within the lowload range (A) may be referred to as “low-load SI combustion.”

When the operating state of the engine 1 is within the low load range(A), the air-fuel ratio (A/F) of the mixture gas is at thestoichiometric air-fuel ratio (A/F≈14.7:1). Note that below, theair-fuel ratio, an excess air ratio λ, and the value of G/F mean thevalues taken at an ignition timing. When the air-fuel ratio of themixture gas is set to the stoichiometric air-fuel ratio, the three-waycatalyst is able to purify the exhaust gas discharged from thecombustion chamber 17, and thus the exhaust gas performance of theengine 1 improves. The A/F of the mixture gas may be set to remainwithin the purification window of the three-way catalyst (i.e., anair-fuel ratio width exhibiting the three-way purification function).Therefore, the excess air ratio λ of the mixture gas may be set to1.0±0.2.

In order to improve the fuel efficiency of the engine 1, when theoperating state of the engine 1 is within the low load range (A), theEGR system 55 introduces the EGR gas into the combustion chamber 17. TheG/F of the mixture gas, which is a mass ratio of the total gas to thefuel in the combustion chamber 17, is set between 18 and 30. The mixturegas is EGR lean and has a high dilution ratio. By setting the G/F of themixture gas to, for example, 25, within the low load range (A), the SIcombustion is stably performed without the mixture gas self-igniting.Within the low load range (A), the G/F of the mixture gas is maintainedsubstantially constant regardless of the engine load. Thus, the SIcombustion is stable throughout the entire low load range. Additionally,the fuel efficiency of the engine 1 improves and the exhaust gasperformance improves.

When the operating state of the engine 1 is within the low load range(A), since the fuel amount is low, a charge amount of gas into thecombustion chamber 17 needs to be lower than 100% in order to bring λ,of the mixture gas to 1.0±0.2 and G/F to a value between 18 and 30. Forexample, the engine 1 executes throttling for adjusting the opening ofthe throttle valve 43 and/or a mirror cycle for retarding the closetiming of the intake valve 21 to after a bottom dead center (BDC) on theintake stroke.

Note that within a low-load and low-speed segment of the low load range(A), the combustion temperature of the mixture gas and the temperatureof the exhaust gas may be raised by reducing the charge amount of gaseven smaller. This is advantageous in keeping the catalytic converter inan active state.

(Medium Load Range)

When the operating state of the engine 1 is within the medium load range(B), the fuel injection amount increases. The temperature of thecombustion chamber 17 increases, and thus, the self-ignition isperformed stably. Within the medium load range (B), the engine performsthe CI combustion in order to improve the fuel efficiency and exhaustgas performance.

In the combustion caused by self-ignition, the timing of theself-ignition changes greatly if the temperature inside the combustionchamber varies before the compression starts. Therefore, within themedium load range (B), the engine 1 performs the SPCCI combustion inwhich the SI combustion and the CI combustion are combined. In the SPCCIcombustion, the spark plug 25 forcibly ignites the mixture gas insidethe combustion chamber 17 to combust it through flame propagation, andthe heat generated by this combustion raises the temperature inside thecombustion chamber 17, which leads to combustion of unburned mixture gasby self-ignition. It is possible to reduce the variation of thetemperature inside the combustion chamber 17 before the compressionstarts by adjusting the heat generation amount in the SI combustion.Therefore, even when the variation in the temperature inside thecombustion chamber 17 varies before the compression starts, for example,by controlling the ignition timing to adjust the start timing of the SIcombustion, the unburned mixture gas self-ignites at a target timing.

In order to accurately control the timing of self-ignition in the SPCCIcombustion, the self-ignition timing needs to change corresponding tothe change of the ignition timing. It is preferable that the sensitivityof the self-ignition timing changing according to the change of theignition timing is high.

According to a study conducted by the present inventors, it was foundthat when the G/F of the mixture gas is between 18 and 30, the SPCCIcombustion is stably performed and the self-ignition timing sensitivelychanges in response to the change of the ignition timing. Therefore,when the operating state of the engine 1 is within the medium load range(B), the engine 1 sets the state inside the combustion chamber 17 sothat λ, of the mixture gas becomes 1.0±0.2 and the G/F of the mixturegas becomes a value between 18 and 30. Moreover, at the ignition timing,a required temperature Tig inside the combustion chamber 17 is 570 to800 K, a required pressure Pig inside the combustion chamber 17 is 400to 920 kPa, and turbulence kinetic energy inside the combustion chamber17 is 17 to 40 m²/s².

By accurately controlling the self-ignition timing in the SPCCIcombustion, an increase of the combustion noise is avoided when theoperating state of the engine 1 is within the medium load range (B).Moreover, by increasing the dilution ratio of the mixture gas as high aspossible and performing the CI combustion, the fuel efficiency of theengine 1 is improved. Moreover, by setting λ of the mixture gas to1.0±0.2, the three-way catalyst is able to purify the exhaust gas, andthus the exhaust gas performance of the engine 1 improves.

As described above, within the low load range (A), the G/F of themixture gas is set between 18 and 30 (e.g., 25) and λ of the mixture gasis set to 1.0±0.2. The state function inside the combustion chamber 17does not vary greatly between the states where the operating state ofthe engine 1 is within the low load range (A) and within the medium loadrange (B). Therefore, robustness of the control of the engine 1 againstthe change of the engine load improves.

When the operating state of the engine 1 is within the medium load range(B), different from being within the low load range (A), the fuel amountincreases, therefore the charge amount of gas introduced into thecombustion chamber 17 is not required to be adjusted. Here, the throttlevalve 43 is fully opened.

When the engine load increases and the fuel amount further increases, inthe naturally aspirated state, the introduction amount of gas into thecombustion chamber 17 becomes insufficient for setting λ of the mixturegas to 1.0±0.2 and the G/F of the mixture gas between 18 and 30.Therefore, in a segment of the medium load range (B) where the engineload is higher than a given load, the booster 44 boosts the gas to beintroduced into the combustion chamber 17. The medium load range (B) isdivided into a first medium load segment (B1) in which the engine loadis higher than the given load and the boost is performed, and a secondmedium load segment (B2) in which the engine load is lower than thegiven load and the boost is not performed. The given load is, forexample, ½ load. The second medium load segment (B2) is a segment wherethe engine load is lower than the first medium load segment (B1).Hereinafter, the combustion mode within the first medium load segment(B1) may be referred to as “boosted SPCCI combustion” and the combustionmode within the second medium load segment (B2) may be referred to as“non-boosted SPCCI combustion.”

Within the second medium load segment (B2) in which the boost is notperformed, as the fuel amount increases, the introduction amount offresh air into the combustion chamber 17 increases while the EGR gasdecreases. The G/F of the mixture gas decreases as the engine loadincreases. Since the throttle valve 43 is fully opened, the engine 1adjusts the introduction amount of EGR gas into the combustion chamber17 to adjust the amount of fresh air introduced into the combustionchamber 17. Within the second medium load segment (B2), the statefunction inside the combustion chamber 17 is set such that, for example,λ of the mixture gas is substantially constant at 1.0 while the G/F ofthe mixture gas is changed between 25 and 28.

On the other hand, within the first medium load segment (B1) in whichthe boost is performed, the engine 1 increases the introduction amountsof fresh air and EGR gas into the combustion chamber 17 as the fuelamount increases. Thus, the G/F of the mixture gas is substantiallyconstant even when the engine load increases. In the state functioninside the combustion chamber 17 within the first medium load segment(B1), for example, λ of the mixture gas is substantially constant at 1.0and the G/F of the mixture gas is constant at 25.

(High Load Range)

The combustion mode when the operating state of the engine 1 is withinthe high load range (C) is the SI combustion. This is for prioritizingavoiding the combustion noise. Hereinafter, the combustion mode withinthe high load range (C) may be referred to as “high-load SI combustion.”

When the operating state of the engine 1 is within the high load range(C), λ of the mixture gas becomes 1.0±0.2, and the G/F of the mixturegas is basically set at between 18 and 30. Within the high load range(C), the throttle valve 43 is fully opened and the booster 44 performsthe boost.

Within the high load range (C), the engine 1 reduces the EGR gas amountas the engine load increases. The G/F of the mixture gas decreases asthe engine load increases. The introduction amount of fresh air into thecombustion chamber 17 increases by the reduced amount of EGR gas,therefore, the fuel amount may be increased, which is advantageous inincreasing a highest output of the engine 1.

The state function inside the combustion chamber 17 does not varygreatly between the states where the operating state of the engine 1 iswithin the high load range (C) and within the medium load range (B).Therefore, the robustness of the control of the engine 1 against thechange of the engine load improves.

Since the engine 1 performs the SI combustion within the high load range(C) as described above, there is an issue with abnormal combustion, suchas pre-ignition and knocking, occurring easily.

Therefore, within the high load range (C), by devising the fuelinjection mode, abnormal combustion is avoided in the engine 1. Forexample, the ECU 10 outputs control signals to the fuel supply system 61and the injector 6 to inject the fuel into the combustion chamber 17 ata fuel pressure of 30 MPa or higher, at a timing in a period from afinal stage of the compression stroke to an early stage of the expansionstroke (hereinafter, this period is referred to as “retard period”). TheECU 10 also outputs a control signal to the spark plug 25 to ignite themixture gas at a timing near CTDC after the fuel injection. Hereinafter,the fuel injection into the combustion chamber 17 at the high fuelpressure at the timing in the retard period is referred to as“high-pressure retard injection.”

The high-pressure retard injection shortens reaction time of the mixturegas to avoid abnormal combustion. That is, the reaction time of themixture gas is a total length of time of (1) a period for which theinjector 6 injects the fuel (i.e., injection period), (2) a period forwhich combustible mixture gas is formed around the spark plug 25 afterthe fuel injection (i.e., mixture gas formation period), and (3) aperiod from the start of ignition until the SI combustion ends (i.e.,combustion period).

The injection period and the mixture gas formation period become shorterby injecting the fuel into the combustion chamber 17 at the high fuelpressure. By shortening the injection period and the mixture gasformation period, the timing of starting the fuel injection approachesthe ignition timing. In the high-pressure retard injection, since thefuel injection into the combustion chamber 17 is performed at the highfuel pressure, the fuel is injected at a timing in the retard periodfrom the final stage of the compression stroke to the early stage of theexpansion stroke.

Injecting the fuel into the combustion chamber 17 at the high fuelpressure increases turbulence kinetic energy inside the combustionchamber 17. By bringing the fuel injection timing close to CTDC, it ispossible to start the SI combustion while the turbulence kinetic energyinside the combustion chamber 17 is high. As a result, the combustionperiod becomes short.

Thus, in the high-pressure retard injection, since the injection period,the mixture gas formation period, and the combustion period arerespectively shortened, the reaction time of the mixture gas issignificantly shortened compared with a case where the fuel is injectedinto the combustion chamber 17 on the intake stroke. As a result,abnormal combustion is avoided.

In the technical field of the engine control, conventionally, theignition timing is retarded to avoid abnormal combustion. However,retarding the ignition timing degrades the fuel efficiency. In thehigh-pressure retard injection, the ignition timing is not required tobe retarded. Therefore, the fuel efficiency improves by using thehigh-pressure retard injection.

By setting the fuel pressure to be, for example, 30 MPa or higher, theinjection period, the mixture gas formation period, and the combustionperiod are effectively shortened. Note that the fuel pressure maysuitably be set according to properties of the fuel. An upper limit ofthe fuel pressure may be, for example, 120 MPa.

Here, when the engine speed is low, compared to when it is high, thetime required for the crank angle to change by the same angle is longer,therefore, shortening the reaction time of the mixture gas by thehigh-pressure retard injection is particularly effective in avoidingabnormal combustion. On the other hand, when the engine speed is high,due to the shorter time required for the crank angle to change by thesame angle, shortening the reaction time of the mixture gas is notparticularly effective in avoiding abnormal combustion.

Further in the high-pressure retard injection, the fuel is injected intothe combustion chamber 17 only after reaching near CTDC, on thecompression stroke, fuel-free gas, in other words, the gas with a highratio of specific heat, is compressed within the combustion chamber 17.If the high-pressure retard injection is performed when the engine speedis high, the temperature inside the combustion chamber 17 at CTDC, i.e.,the compression end temperature, rises, which may cause abnormalcombustion, such as knocking.

Therefore, in the engine 1, the high load range (C) is divided into afirst high load segment (C1) on the low engine speed side and a secondhigh load segment (C2) where the engine speed is higher than within thefirst high load segment (C1). When the high load range (C) is evenlydivided into three ranges of low engine speed, medium engine speed, andhigh engine speed, the first high load segment (C1) may include the lowengine speed range and the medium engine speed range, and the secondhigh load segment (C2) may include the high engine speed range.

Within the first high load segment (C1), the injector 6, in response toreceiving the control signal of the ECU 10, performs the high-pressureretard injection described above. Within the second high load segment(C2), the injector 6, in response to receiving the control signal of theECU 10, performs the fuel injection at a given timing on the intakestroke. The fuel injection performed on the intake stroke does notrequire high fuel pressure. Therefore, the ECU 10 outputs the controlsignal to the fuel supply system 61 so that the fuel pressure fallsbelow the fuel pressure of the high-pressure retard injection (e.g.,below 40 MPa). Since lowering the fuel pressure reduces a mechanicalresistance loss of the engine 1, it is advantageous in improving thefuel efficiency.

The ratio of the specific heat of the gas inside the combustion chamber17 decreases by injecting the fuel into the combustion chamber 17 on theintake stroke, therefore, the compression end temperature drops, andthus, abnormal combustion is avoided. Since it is not necessary toretard the ignition timing for avoiding abnormal combustion, within thesecond high load segment (C2), similar to the first high load segment(C1), the spark plug 25 ignites the mixture gas at a timing near CTDC.

Within the first high load segment (C1), since the mixture gas does notresult in self-ignition because the high-pressure retard injection isapplied, the engine 1 performs stable SI combustion. Within the secondhigh load segment (C2), since the mixture gas does not result inself-ignition because the fuel is injected on the intake stroke, theengine 1 performs stable SI combustion.

(SPCCI Combustion)

Next, the SPCCI combustion described above is described in detail. Theupper chart of FIG. 8 illustrates a waveform 801 which is one example ofa change in a heat generation rate with respect to the crank angle. Whenthe spark plug 25 ignites the mixture gas near CTDC, specifically at agiven timing before CTDC, the combustion starts by flame propagation.The heat generation in the SI combustion is slower than the heatgeneration in the CI combustion. Therefore, the waveform of the heatgeneration rate has a relatively shallow slope. Although notillustrated, a pressure variation (dp/dθ) in the combustion chamber 17is shallower in the SI combustion than in the CI combustion.

When the temperature and pressure inside the combustion chamber 17 risedue to the SI combustion, the unburned mixture gas self-ignites. In theexample of the waveform 801, the slope of the waveform of the heatgeneration rate changes from gentle to sharp, i.e., the waveform of theheat generation rate has a flexion point at a timing when the CIcombustion starts.

After the CI combustion starts, the SI combustion and the CI combustionare performed in parallel. In the CI combustion, since the heatgeneration is greater than in the SI combustion, the heat generationrate becomes relatively high. Note that since the CI combustion isperformed after CTDC, the piston 3 descends by motoring. Therefore, theslope of the waveform of the heat generation rate by the CI combustionis avoided from becoming excessively sharp. The dp/dθ in the CIcombustion also becomes relatively gentle.

The dp/dθ is usable as an index expressing the combustion noise. Sincethe SPCCI combustion is able to lower the dp/dθ as described above, itbecomes possible to avoid the combustion noise from becoming excessivelyloud. Thus, combustion noise is suppressed below an allowable value.

The SPCCI combustion ends by finishing the CI combustion. The CIcombustion has a shorter combustion period than in the SI combustion.The SPCCI combustion advances the combustion end timing compared to theSI combustion. In other words, the SPCCI combustion brings thecombustion end timing on the expansion stroke closer to CTDC. The SPCCIcombustion is advantageous in improving fuel efficiency of the engine 1compared to the SI combustion.

Therefore, the SPCCI combustion achieves both prevention of thecombustion noise and improvement in the fuel efficiency.

Here, an SI ratio is defined as a parameter indicating a property of theSPCCI combustion. The SI ratio is defined as an index relating to aratio of the heat amount generated by the SI combustion with respect toa total heat amount generated by the SPCCI combustion. The SI ratio is aheat volume ratio resulted from two combustions with differentcombustion modes. The SI ratio may be a ratio of heat amount generatedby the SI combustion with respect to the heat amount generated by theSPCCI combustion. For example, in the waveform 801, the SI ratio may beexpressed as SI ratio=(area of SI combustion)/(area of SPCCIcombustion). In the waveform 801, the SI ratio may be referred to as “SIfuel ratio” in the meaning of the ratio of fuel to be combusted in theSI combustion.

In the SPCCI combustion combined the SI combustion and the CIcombustion, the SI ratio is a ratio of the SI combustion with respect tothe CI combustion. The ratio of the SI combustion is high when the SIratio is high, and the ratio of the CI combustion is high when the SIratio is low.

Various definitions may be considered for the SI ratio without limitingto the definition described above. For example, the SI ratio may be aratio of the heat amount generated by the SI combustion with respect tothe heat amount generated by the CI combustion. In other words, in thewaveform 801, SI ratio=(area of SI combustion)/(area of CI combustion)may be set.

Further, in the SPCCI combustion, the waveform of the heat generationrate has a flexion point at the timing when the CI combustion starts.Therefore, as indicated by the reference character 802 in the middlechart of FIG. 8, by having a boundary at the flexion point in thewaveform of the heat generation rate, the SI combustion may be appliedfor a range on the advancing side of the boundary, and the CI combustionmay be applied for a range on the retarding side of the boundary. Inthis case, as indicated by hatching the waveform 802, based on an areaQ_(SI) of the advancing-side range and an area Q_(CI) of theretarding-side range, SI ratio=Q_(SI)/(Q_(SI)+Q_(CI)) or SIratio=Q_(SI)/Q_(CI) may be set. Alternatively, the SI ratio may bedefined based on an area of a part of the advancing-side range and anarea of a part of the retarding-side range, instead of the entire area.

Further, instead of defining the SI ratio based on the heat generation,based on a crank angle Δθ_(SI) of the advancing-side range and a crankangle Δθ_(CI) of the retarding-side range, SIratio=Δθ_(SI)/(Δθ_(SI)+Δθ_(CI)) or SI ratio=Δθ_(SI)/Δθ_(CI) may be set.

Moreover, based on a peak Δβ_(SI) of the heat generation rate in theadvancing-side range and a peak ΔP_(CI) of the heat generation rate inthe retarding-side range, SI ratio=ΔP_(SI)/(ΔP_(SI)+ΔP_(CI)) or SIratio=ΔP_(SI)/ΔP_(CI) may be set.

Furthermore, based on a slope φ_(SI) of the heat generation rate in theadvancing-side range and a slope φ_(CI) of the heat generation rate inthe retarding-side range, SI ratio=φ_(SI)/(φ_(SI)+φ_(CI)) or SIratio=φ_(SI)/φ_(CI) may be set.

Additionally, in this embodiment, the SI ratio is defined by one of thearea (i.e., the heat generation amount), length in the horizontal axis(i.e., the crank angle), length in the vertical axis (i.e., the heatgeneration rate), and the slope (i.e., the change rate in the heatgeneration rate) based on the waveform of the heat generation rate.Although not illustrated, the SI ratio may similarly be defined based ona waveform of pressure (P) in the combustion chamber 17, by one of thearea, length in the horizontal axis, length in the vertical axis, andthe slope.

In the SPCCI combustion, the flexion point of the combustion waveformregarding the heat generation rate or pressure does not necessarilyappear clearly all the time. The following may be used as a definitionof the SI ratio which is not based on the flexion point. That is, asindicated by a reference character 803 in the lower chart of FIG. 8, inthe combustion waveform, the SI combustion may be applied for a range onthe advancing side of CTDC and the CI combustion may be applied for arange on the retarding side of CTDC. Under this condition, the SI ratiomay be defined by one of the area (Q_(SI), Q_(CI)), length in thehorizontal axis (Δθ_(SI), Δθ_(CI)), length in the vertical axis(ΔP_(SI), ΔP_(CI)) and the slope ((φ_(SI), φ_(CI)).

Alternatively, the SI ratio may be defined based on the fuel amountinstead of the actual combustion waveform in the combustion chamber 17.As described later, within the medium load range in which the SPCCIcombustion is performed, split injections including a first-stageinjection and a second-stage injection may be performed. The fuelinjected into the combustion chamber 17 by the second-stage injectionignites within a short time after the injection, it reaches near thespark plug 25 without spreading inside the combustion chamber 17.Therefore, the fuel injected into the combustion chamber 17 by thesecond-stage injection combusts mainly in the SI combustion. On theother hand, the fuel injected into the combustion chamber 17 by thefirst-stage injection combusts mainly in the CI combustion. Therefore,the SI ratio may be defined based on the fuel amount injected in thefirst-stage injection (m₁) and the fuel amount injected in thesecond-stage injection (m₂). In other words, SI ratio=m₂/(m₁+m₂) or SIratio=m₂/m₁ may be set.

(Operation Control of Engine in Load Direction)

As described above, the engine 1 switches between the SI combustion andthe SPCCI combustion according to the operating state. Further, theengine 1 changes the SI ratio according to the operating state of theengine 1. Thus, the engine 1 is achieved in preventing the generation ofcombustion noise and improving the fuel efficiency.

FIG. 9 is a chart illustrating a change in the SI ratio, a change in thestate function inside the combustion chamber 17, changes in the openperiods of the intake valve 21 and the exhaust valve 22, and changes inthe injection timing and ignition timing of the fuel, with respect tothe engine load. FIG. 9 corresponds to the operating range map 700 ofFIG. 7A. Hereinafter, the operation control of the engine 1 is describedfor a condition in which the engine load gradually increases at a givenengine speed.

(Low Load Range (Low-Load SI Combustion))

Within the low load range (A), the engine 1 performs the low-load SIcombustion. When the operating state of the engine 1 is within the lowload range (A), the SI ratio is constant at 100%.

Within the low load range (A), as described above, the G/F of themixture gas is fixed between 18 and 30. The engine 1 introduces thefresh air and the burned gas by amounts corresponding to the fuelamount, into the combustion chamber 17. The introduction amount of freshair, as described above, is adjusted by throttling and/or the mirrorcycle. Since the dilution ratio is high, the temperature inside thecombustion chamber 17 is raised to stabilize the SI combustion. Withinthe low load range (A), the engine 1 introduces the internal EGR gasinto the combustion chamber 17.

The internal EGR gas is introduced into the combustion chamber 17 (i.e.,the burned gas is confined inside the combustion chamber 17) byproviding a negative overlap period in which the intake and exhaustvalves 21 and 22 are both closed over the exhaust TDC. The adjustment ofthe internal EGR gas amount is performed by suitably setting the lengthof the negative overlap period by the intake electrically-operated S-VT23 adjusting the open timing of the intake valve 21 and the exhaustelectrically-operated S-VT 24 adjusting the open timing of the exhaustvalve 22. Note that the internal EGR gas may be introduced into thecombustion chamber 17 by providing a positive overlap period in whichthe intake and exhaust valves 21 and 22 are both opened.

Within the low load range (A), the charge amount into the combustionchamber 17 is adjusted to be below 100%. The amount of fresh airintroduced into the combustion chamber 17 and the amount of the internalEGR gas gradually increase as the fuel amount increases. The EGR ratiowithin the low load range (A) is, for example, 40%.

The injector 6 injects the fuel into the combustion chamber 17 on theintake stroke. Inside the combustion chamber 17, a homogeneous mixturegas in which the excess air ratio λ is 1.0±0.2 and the G/F is between 18and 30 is formed. The excess air ratio λ is preferably 1.0 to 1.2. Bythe spark plug 25 igniting the mixture gas at the given timing beforeCTDC, the mixture gas combusts by flame propagation without reaching theself-ignition.

(Second Medium Load Segment (Non-Boosted SPCCI Combustion))

When the engine load increases and the operating state enters the secondmedium load segment (B2), the engine 1 switches from the low-load SIcombustion to the non-boosted SPCCI combustion. The SI ratio falls below100%. The fuel amount increases as the engine load increases. When theengine load is low within the second medium load segment (B2), the ratioof the CI combustion is increased as the fuel amount increases. The SIratio gradually decreases as the engine load increases. In the exampleof FIG. 9, the SI ratio decreases to a given value (lowest value) lowerthan 50%.

Since the fuel amount increases, the combustion temperature rises withinthe second medium load segment (B2). If the temperature inside thecombustion chamber 17 rises excessively, the heat generation at thestart of the CI combustion becomes sharp, which results in increasingthe combustion noise.

Therefore, within the second medium load segment (B2), the ratio betweenthe internal EGR gas and the external EGR gas is changed according tothe change in the engine load in order to adjust the temperature insidethe combustion chamber 17 before the compression starts. That is, as theengine load increases, the internal EGR gas is gradually reduced and thecooled external EGR gas is gradually increased. Within the second mediumload segment (B2), the negative overlap period changes from a longestlength to zero as the engine load increases. Also within the secondmedium load segment (B2), the internal EGR gas becomes zero when theengine load reaches a highest value. This is similar for when thepositive overlap period between the intake and exhaust valves 21 and 22is provided. As a result of control the temperature inside thecombustion chamber 17 by adjusting the overlap period, the SI ratio inthe SPCCI combustion is adjusted.

Within the second medium load segment (B2), the opening of the EGR valve54 is changed so that the external EGR gas increases as the engine loadincreases. The amount of external EGR gas introduced into the combustionchamber 17 is adjusted, when expressed by the EGR ratio, between 0 and30%, for example. Within the second medium load segment (B2), the EGRgas is replaced from the internal EGR gas to the external EGR gas as theengine load increases. Since the temperature inside the combustionchamber 17 is also controlled by adjusting the EGR ratio, the SI ratioof the SPCCI combustion is adjusted.

Note that the EGR gas amount introduced into the combustion chamber 17is continuous between the low load range (A) and the second medium loadsegment (B2). Within a low engine load section of the second medium loadsegment (B2), a large amount of the internal EGR gas is introduced intothe combustion chamber 17, the same as in the low load range (A). Sincethe temperature inside the combustion chamber 17 becomes high, themixture gas surely self-ignites when the engine load is low. Within ahigh engine load section of the second medium load segment (B2), theexternal EGR gas is introduced into the combustion chamber 17. Since thetemperature inside the combustion chamber 17 decreases, the combustionnoise accompanying the CI combustion is reduced when the engine load ishigh.

Within the second medium load segment (B2), the charge amount introducedinto the combustion chamber 17 is set to 100%. The throttle valve 43 isfully opened. By adjusting the EGR gas amount which is a total of theinternal EGR gas and the external EGR gas, the introduction amount offresh air into the combustion chamber 17 is adjusted to the amountcorresponding to the fuel amount.

As the ratio of the CI combustion in the non-boosted SPCCI combustionincreases, the self-ignition timing advances. If the self-ignitiontiming advances than CTDC, the heat generation at the start of the CIcombustion becomes sharp, which results in increasing the combustionnoise. Therefore, in the engine 1, once the engine load reaches thegiven load L1, the SI ratio is gradually increased as the engine loadfurther increases therefrom.

That is, the engine 1 increases the ratio of the SI combustion as thefuel amount increases. For example, as illustrated in the upper chart ofFIG. 10, in the non-boosted SPCCI combustion, the ignition timing isgradually advanced as the fuel amount increases. Since the temperatureinside the combustion chamber 17 is adjusted by reducing theintroduction amount of the internal EGR gas and increasing theintroduction amount of the external EGR gas as described above, thetemperature rise at CTDC is suppressed even when the SI ratio isincreased as the fuel amount increases. The slope of the heat generationrate of the SI combustion scarcely changes even when the engine loadincreases. Advancing the ignition timing causes the SI combustion tostart earlier, and the heat generation amount of SI combustionaccordingly increases.

As a result of suppressing the temperature rise inside the combustionchamber 17 caused by the SI combustion, the unburned mixture gasself-ignites at a timing after CTDC. The heat generation amount by theCI combustion is substantially the same even when the engine loadincreases since the heat generation amount of the SI combustion isincreased. Therefore, by setting the SI ratio to be gradually higheraccording to the increase in the engine load, the combustion noise isavoided from increasing. Note that the center of gravity of combustionin the non-boosted SPCCI combustion retards as the engine loadincreases.

Within the second medium load segment (B2), the injector 6 injects thefuel into the combustion chamber 17 in two stages, the first-stageinjection and the second-stage injection. In the first-stage injection,the fuel is injected at a timing separated from the ignition timing, andin the second-stage injection, the fuel is injected at a timing close tothe ignition timing. The first-stage injection may be performed in aperiod from the intake stroke to an early half of the compressionstroke, and the second-stage injection may be performed in a period froma latter half of the compression stroke to an early half of theexpansion stroke. The early half and latter half of the compressionstroke may be defined by evenly dividing the compression stroke into twoin terms of the crank angle. The early half of the expansion stroke maybe defined by evenly dividing the expansion stroke into two in terms ofthe crank angle.

When the injector 6 performs the first-stage injection in the periodfrom the intake stroke to the early half of the compression stroke,since the piston 3 is separated from TDC, the injected fuel sprayreaches the upper surface of the piston 3 elevating toward TDC, outsidethe cavity 31. A section outside the cavity 31 forms a squish area 171as illustrated in FIG. 2. The fuel injected in the first-stage injectionremains in the squish area 171 while the piston 3 elevates and forms themixture gas in the squish area 171. This mixture gas is combusted mainlyin the CI combustion.

When the injector 6 performs the second-stage injection in the periodfrom the latter half of the compression stroke to the early half of theexpansion stroke, since the piston 3 is close to TDC, the injected fuelspray enters the cavity 31. The fuel injected in the second-stageinjection forms the mixture gas in the section within the cavity 31.Here, the phrase “the section within the cavity 31” may mean acombination of a section from a projection plane of the opening surfaceof the cavity on the ceiling surface of the combustion chamber 17 to theopening surface of the cavity 31 and a section inside the cavity 31. Thephrase “the section within the cavity 31” may also be said to be asection of the combustion chamber 17 outside the squish area 171. Thefuel is distributed substantially evenly inside the entire combustionchamber 17 by the first-stage injection and the second-stage injection.

Due to injecting the fuel into the cavity 31 by the second-stageinjection, the flow of gas occurs in the section within the cavity 31.When the time to the ignition timing is long, the turbulence kineticenergy inside the combustion chamber 17 attenuates as the compressionstroke progresses. However, since the injection timing of thesecond-stage injection is close to the ignition timing compared to thatof the first-stage injection, the spark plug 25 ignites the mixture gasin the section within the cavity 31 while keeping the high turbulencekinetic energy therewithin. Thus, the speed of the SI combustionincreases. Since the SI combustion becomes stable when the speed of theSI combustion increases, the controllability of the CI combustion by theSI combustion improves.

In the entire combustion chamber 17, the mixture gas becomes a statewhere the excess air ratio λ is 1.0±0.2 and the G/F is a value between18 and 30. Since the fuel is distributed substantially homogeneously,the improvement in fuel efficiency by reducing an unburned fuel loss andthe improvement in exhaust gas performance by avoiding the smokegeneration are achieved. Note that the excess air ratio λ is preferably1.0 to 1.2 in the entire combustion chamber 17.

By the spark plug 25 igniting the mixture gas at the given timing beforeCTDC, the mixture gas combusts by flame propagation. Then the unburnedmixture gas self-ignites at the target timing and causes the CIcombustion. The fuel injected in the second-stage injection mainlycauses the SI combustion. The fuel injected in the first-stage injectionmainly causes the CI combustion. Since the first-stage injection isperformed on the compression stroke, the fuel injected in thefirst-stage injection is prevented from causing abnormal combustion,such as the pre-ignition. Moreover, the fuel injected in thesecond-stage injection is stably combusted by flame propagation.

(First Medium Load Segment (Boosted SPCCI Combustion))

When the engine load further increases and the operating state of theengine 1 enters the first medium load segment (B1), the booster 44boosts the fresh air and the external EGR gas. The amount of fresh airintroduced into the combustion chamber 17 and the amount of the externalEGR gas both increase as the engine load increases. The amount ofexternal EGR gas introduced into the combustion chamber 17 is, whenexpressed by the EGR ratio, 30%, for example. The EGR ratio issubstantially constant regardless of the engine load. Therefore, the G/Fof the mixture gas is also substantially constant regardless of theengine load. Note that the EGR gas amount introduced into the combustionchamber 17 is continuous between the second medium load segment (B2) andthe first medium load segment (B1).

The SI ratio is fixed or substantially fixed at a given value below 100%with respect to the engine load. When the SI ratio of the second mediumload segment (B2), particularly the SI ratio which gradually increasesas the engine load increases from a value above the given load L1 iscompared with the SI ratio of the first medium load segment (B1), the SIratio of the first medium load segment (B1) where the engine load ishigher is higher than that of the second medium load segment (B2). TheSI ratio is continuous over the boundary between the first medium loadsegment (B1) and the second medium load segment (B2).

Here, within the first medium load segment, the SI ratio may slightly bechanged according to the change of the engine load. The change rate ofthe SI ratio according to the engine load within the first medium loadsegment may be lower than that at a high engine load side of the secondmedium load segment.

As illustrated in the lower chart of FIG. 10, also in the boosted SPCCIcombustion, the ignition timing is gradually advanced as the fuel amountincreases. Since the fresh air and the EGR gas amount introduced intothe combustion chamber 17 are increased by boosting as described above,the heat volume is large. Therefore, even when the fuel amountincreases, the temperature increase inside the combustion chamber 17caused by the SI combustion is suppressed. The waveform of the heatgeneration rate of the boosted SPCCI combustion becomes larger (the areaof the section formed by the waveform and the horizontal axis becomeslarger) in a similar shape as the engine load increases.

That is, the heat generation amount of the SI combustion increases whilethe slope of the heat generation rate of the SI combustion scarcelychanges. The unburned mixture gas self-ignites at substantially the sametiming after CTDC. The heat generation amount of the CI combustionincreases as the engine load increases. As a result, within the firstmedium load segment (B1), since both the heat generation amount of theSI combustion and the heat generation amount of the CI combustionincrease, the SI ratio is constant with respect to the engine load.Although the combustion noise increases when the peak of the heatgeneration of the CI combustion rises, since the engine load isrelatively high within the first medium load segment (B1), a certainlevel of combustion noise is allowed. Note that the center of gravity ofcombustion in the boosted SPCCI combustion retards as the engine loadincreases.

Within the first medium load segment (B1), the positive overlap periodin which the intake and exhaust valves 21 and 22 are both opened isprovided over the exhaust TDC. The unburned gas residing in thecombustion chamber 17 is scavenged by the boosting pressure. Thus, thetemperature in the combustion chamber 17 drops, and as a result,abnormal combustion is prevented from occurring when the engine load isrelatively high. Further, by dropping the temperature in the combustionchamber 17, within the range where the engine load is relatively high,the self-ignition timing is adjusted to a suitable timing and the SIratio is maintained at a given SI ratio. That is, the SI ratio iscontrolled by adjusting the overlap period. Further, by scavenging theburned gas, the charge amount of fresh air in the combustion chamber 17is increased.

Within the first medium load segment (B1), similarly to the secondmedium load segment (B2), the injector 6 injects the fuel into thecombustion chamber 17 in two stages, the first-stage injection and thesecond-stage injection. In the first-stage injection, the fuel isinjected at the timing separated from the ignition timing, and in thesecond-stage injection, the fuel is injected at the timing close to theignition timing. The first-stage injection may be performed in theperiod from the intake stroke to the early half of the compressionstroke, and the second-stage injection may be performed in the periodfrom the latter half of the compression stroke to the early half of theexpansion stroke.

When the injector 6 performs the first-stage injection in the periodfrom the intake stroke to the early half of the compression stroke, themixture gas is formed in the squish area 171. When the injector 6performs the second-stage injection in the period from the latter halfof the compression stroke to the early half of the expansion stroke, themixture gas is formed in the cavity 31.

By the injector 6 performing the first-stage injection and thesecond-stage injection, in the combustion chamber 17, substantiallyhomogeneous mixture gas in which the excess air ratio λ is 1.0±0.2 andthe G/F is 18 to 30 is formed as a whole. Since the mixture gas issubstantially homogeneous, the improvement in the fuel efficiency byreducing the unburned fuel loss and the improvement in the exhaust gasperformance by avoiding the smoke generation are achieved. Note that theexcess air ratio λ is preferably 1.0 to 1.2 in the entire combustionchamber 17.

By the spark plug 25 igniting the mixture gas at the given timing beforeCTDC, the mixture gas combusts by flame propagation. Then the unburnedmixture gas self-ignites at the target timing and causes the CIcombustion. The fuel injected in the second-stage injection mainlycauses the SI combustion. The fuel injected in the first-stage injectionmainly causes the CI combustion. Since the first-stage injection isperformed on the compression stroke, the fuel injected in thefirst-stage injection is prevented from causing abnormal combustion,such as the pre-ignition. Moreover, the fuel injected in thesecond-stage injection is stably combusted by flame propagation.

(High Load Range (High-Load SI Combustion))

When the engine load increases and the operating state of the engine 1enters the high load range (C), the engine 1 performs the high-load SIcombustion. Therefore, the SI ratio within the high load range (C)becomes 100%.

The throttle valve 43 is fully opened. The booster 44 boosts the freshair and the external EGR gas even within the high load range (C). TheEGR valve 54, by adjusting its opening, gradually reduces theintroduction amount of the external EGR gas as the engine loadincreases. In this manner, the fresh air introduced into the combustionchamber 17 increases as the engine load increases. Since the fuel amountis increased as the fresh air amount increases, it is advantageous inincreasing the highest output of the engine 1. Note that the EGR gasamount introduced into the combustion chamber 17 is continuous betweenthe first medium load segment (B1) and the high load range (C).

Also within the high load range (C), similarly to the first medium loadsegment (B1), the positive overlap period in which the intake andexhaust valves 21 and 22 are both opened is provided over the exhaustTDC. The unburned gas residing in the combustion chamber 17 is scavengedby the boosting pressure. Thus, the occurrence of abnormal combustion isprevented. Further, the charge amount of fresh air in the combustionchamber 17 is increased.

Within a low engine speed segment of the high load range (C) (i.e., thefirst high load segment (C1)), the injector 6 injects the fuel into thecombustion chamber 17 in the retard period as described above. Within ahigh engine speed segment of the high load range (C) (i.e., the secondhigh load segment (C2)), the injector 6 injects the fuel into thecombustion chamber 17 on the intake stroke. Within either segment,substantially homogeneous mixture gas in which the excess air ratio λ is1.0±0.2 and the G/F is 18 to 30 is formed inside the combustion chamber17. At the highest load, the excess air ratio λ may be 0.8, for example.Moreover, the G/F of the mixture gas may be, for example, 17 at thehighest load. By the spark plug 25 igniting the mixture gas at the giventiming before CTDC, the mixture gas is combusted by flame propagation.Within the high load range (C), due to the high-pressure retardinjection or the fuel injection on the intake stroke, the mixture gascauses the SI combustion without reaching the self-ignition.

(Operation Control of Engine in Speed Direction) (Second-Stage InjectionRatio)

FIG. 11 illustrates a relationship between the engine speed and thesecond-stage injection ratio within the medium load range (B) in whichthe SPCCI combustion is performed. The second-stage injection ratioindicates a ratio of the injection amount in the second-stage injectionwith respect to the injection amount in the first-stage injection. Asthe second-stage injection ratio increases, the injection amount in thesecond-stage injection increases and the injection amount in thefirst-stage injection decreases. On the other hand, as the second-stageinjection ratio decreases, the injection amount in the second-stageinjection decreases and the injection amount in the first-stageinjection increases.

When the engine speed is low, the ECU 10 sets the second-stage injectionratio to a given low injection ratio. As described above, thesecond-stage injection forms the mixture gas around the spark plug 25.This mixture gas is spark-igniting mixture gas which combusts mainly inthe SI combustion of the SPCCI combustion. When the second-stageinjection ratio is low, since the concentration of fuel within thespark-igniting mixture gas decreases, the SI ratio of the SPCCIcombustion decreases and the CI combustion increases. Generally, whenthe engine speed is low, NVH of the engine 1 is low. Therefore, evenwhen the combustion noise increases by a certain level by the CIcombustion, NVH falls below the allowable value. When the engine speedis low, by reducing the second-stage injection ratio and sufficientlyperforming the CI combustion, the fuel efficiency is improved.

When the engine speed increases, NVH of the engine 1 increases.Moreover, with the addition of the combustion noise caused by the CIcombustion, NVH may exceed the allowable value. Therefore, when theengine speed increases, the ECU 10 increases the SI ratio of the SPCCIcombustion. For example, as illustrated in FIG. 14, the ECU 10 linearlyincreases the SI ratio as the engine speed increases. As illustrated inFIG. 7A, a speed N2 corresponds to the boundary between the medium loadrange (B) in which the SPCCI combustion is performed and the high speedrange (D) in which the SI combustion is performed. The SI ratio is 100%at the speed N2.

In order to change the SI ratio according to the change in the enginespeed, the ECU 10 changes the second-stage injection ratio according tothe change in the engine speed as illustrated by a waveform 111 in theupper chart of FIG. 11. For example, when the engine speed exceeds agiven speed N3, the ECU 10 increases the second-stage injection ratio asthe engine speed increases. The given speed N3 is between a lowest speedN1 and the highest speed N2 of the medium load range (B). The givenspeed N3 may be higher than a middle speed ((N1+N2)/2) of the lowestspeed N1 and the highest speed N2 within the medium load range (B).Alternatively, the given speed N3 may be equal to or higher than themiddle speed of the lowest engine speed and the highest engine speed inthe entire operating range of the engine 1 illustrated in FIG. 7A. Inother words, the given speed N3 may suitably be set within the highspeed range when the operating range of the engine 1 is evenly dividedinto two ranges of the low speed range and the high speed range.

In the example of the waveform 111, when the engine speed exceeds thegiven speed N3, the ECU 10 continuously increases the second-stageinjection ratio at a given change rate as the engine speed increases.Alternatively, the ECU 10 may increase in a stepwise fashion (i.e.,discontinuously) the second-stage injection ratio as the engine speedincreases. By increasing the second-stage injection ratio, the fuelconcentration of the spark-ignition mixture gas formed around the sparkplug 25 increases. Since the SI combustion becomes sharp as a result,the SI ratio in the SPCCI combustion increases. Since the CI combustiondecreases as the SI ratio increases, the combustion noise generated bythe SPCCI combustion is reduced. When the engine speed is high, NVH issuppressed below the allowable value.

When the engine 1 is operating in the medium load range (B), the ECU 10outputs a control signal to the injector 6 so that the second-stageinjection ratio changes at the given change rate according to the changein the engine speed as described above. More specifically, the ECU 10sets the change rate when the engine speed is higher than the givenspeed N3 (i.e., the slope of the upper chart of FIG. 11) to be higher(sharper) than the change rate when the engine speed is equal to orlower than the given speed N3 (i.e., in the example of the upper chartof FIG. 11, the slope of the chart is zero).

Note that although not illustrated, when the engine speed is equal to orlower than the given speed N3, instead of setting the change rate of thesecond-stage injection amount to zero, it may be increased as the enginespeed increases. In this case, the change rate when the engine speed isequal to or lower than the given speed N3 may be lower than that whenthe engine speed exceeds the given speed N3.

As illustrated in the upper chart of FIG. 11, an upper limit value isdetermined for the second-stage injection ratio. When the engine speedexceeds a given speed N4, the ECU 10 sets the second-stage injectionratio at the upper limit value. The given speed N4 is lower than thehighest speed N2 of the medium load range (B) illustrated in FIG. 7A. Inthe second-stage injection, since the timing for injecting the fuel interms of the crank angle is late, the period until the injected fuelforms the combustible mixture gas is short. Further, the time for thecrank angle to change by the same angle is shorter as the engine speedincreases. Therefore, the time length from the injection of the fuel inthe second-stage injection to the ignition becomes shorter as the enginespeed increases.

As described above, when the second-stage injection ratio is increasedas the engine speed is increased, a large amount of fuel needs to bevaporized in a short period of time to form the mixture gas. Here, inreality, the amount of fuel which does not combust in the SI combustionof the SPCCI combustion increases, a large amount of fuel causes the CIcombustion, and thus the combustion noise may increase.

Therefore, in a configuration in which the second-stage injection ratiois increased as the engine speed increases, when the engine speedexceeds the given speed N4, the injection amount of the second-stageinjection is limited so as not to exceed a given amount. When the enginespeed exceeds the given speed N4, the ECU 10 fixes the injection amountof the second-stage injection at a given amount. Thus, the fuel which isnot combusted in the SI combustion is prevented from increasing, and thecombustion noise is avoided from increasing due to the CI combustion.

When the second-stage injection ratio is limited to the upper limitvalue, the SI ratio of the SPCCI combustion does not increase, andtherefore, the effect of reducing the combustion noise caused byincreasing the injection amount of the second-stage injection islimited. Therefore, when the second-stage injection ratio is limited tothe upper limit value, the engine 1 is increased in the SI ratio byusing another method. For example, the ECU 10 controls the SCV 56 tostrengthen the flow of the intake air. If the intake air flow isstrengthened, the SI ratio increases because the SI combustion becomessharp. As a result, the combustion noise of the SPCCI combustion isreduced.

A waveform 131 in the upper chart of FIG. 13 illustrates a relationshipbetween the engine speed and the opening of the SCV 56. When the enginespeed reaches the given speed N4 and the second-stage injection ratio islimited to the upper limit value, the ECU 10 narrows the opening of theSCV 56 from the fully opened state. Thus, the swirl flow in thecombustion chamber 17 becomes strong. The ECU 10 linearly changes theopening of the SCV 56 as the engine speed increases. As the engine speedincreases, since the swirl flow becomes stronger, the SI combustionbecomes even sharper. Thus, the combustion noise of the SPCCI combustionis reduced. As a result, when the engine speed is high, NVH issuppressed below the allowable value.

In this engine 1, since NVH is suppressed below the allowable value byadjusting the SI ratio in the speed direction of the engine 1, the rangewhere the SPCCI combustion is performed expands to the higher speedside. Therefore, the engine 1 excels in the fuel efficiency.

When the engine load is high, since the temperature in the combustionchamber 17 becomes relatively high, the SI combustion in the SPCCIcombustion becomes sharper than when the engine load is low. When theengine load is high, the SI ratio becomes higher than when the engineload is low. Therefore, as indicated by the one-dotted chain line ofFIG. 14, the straight line indicating the relationship between theengine speed and the SI ratio has a gentler slope at a high engine loadthan at a low engine load.

When the SI ratio increases due to the high engine load, since thecombustion noise is reduced, it is not required to increase thesecond-stage injection ratio to increase the SI ratio. Therefore, theECU 10 may shift the speed N3 at which the second-stage injection ratiostarts increasing, to the higher speed side, as illustrated by theone-dotted chain line in the upper chart of FIG. 11. In this manner, therange where the low second-stage injection ratio is maintained isexpanded to the higher speed side. As described above, if thesecond-stage injection ratio is low, the CI combustion in the SPCCIcombustion increases, which is advantageous in improving the fuelefficiency.

Note that different from the example of the upper chart of FIG. 11, asindicated by the one-dotted chain line in a waveform 112 of the lowerchart of FIG. 11, the ECU 10 may set the slope of the straight lineindicating the relationship between the engine speed and thesecond-stage injection ratio gentler at a high engine load than at a lowengine load.

(Fuel Injection Timing)

A waveform 121 in the upper chart of FIG. 12 illustrates therelationship between the engine speed and the injection timing of thesecond-stage injection in the medium load range (B) in which the SPCCIcombustion is performed. Note that although not illustrated in FIG. 12,the injection timing of the first-stage injection remains unchanged at agiven timing regardless of the engine speed.

When the engine speed is low, the ECU 10 sets the injection timing ofthe second-stage injection at a given retarded timing. By retarding thetiming of the second-stage injection, the mixture gas is ignited in astate where the flow of the gas in the combustion chamber 17 is strong.Thus, the SI combustion becomes sharp and the self-ignition timing isaccurately controlled.

When the engine speed increases, the vaporization period from theinjection of the fuel in the second-stage injection to the ignitionthereof becomes shorter. In the SPCCI combustion, the amount of themixture gas which does not combust in the SI combustion increases andthe SI ratio drops. As a result, the CI combustion in the SPCCIcombustion increases and the combustion noise of the SPCCI combustionincreases. When the combustion noise increases, NVH may exceed theallowable value.

Therefore, as indicated by the waveform 121, when the engine speedexceeds the given speed N3, the ECU 10 advances the injection timing ofthe second-stage injection at a given change rate as the engine speedincreases. The given speed N3 is the same as the speed N3 illustrated inFIG. 11.

The ECU 10 continuously advances the injection timing of thesecond-stage injection as the engine speed increases. Alternatively, theECU 10 may advance in a stepwise fashion (i.e., discontinuously) theinjection timing of the second-stage injection as the engine speedincreases. By advancing the injection timing of the second-stageinjection, the vaporization period is extended. As a result, the amountof the mixture gas which does not combust in the SI combustiondecreases, which increases the SI ratio in the SPCCI combustion. Asillustrated in FIG. 14, the SI ratio linearly increases as the enginespeed increases. Since the combustion noise of the SPCCI combustion issuppressed low by increasing the SI ratio, when the engine speed ishigh, NVH is suppressed below the allowable value.

When the engine 1 is operating in the medium load range (B), the ECU 10outputs a control signal to the injector 6 so that the second-stageinjection timing changes at the given change rate according to thechange in the engine speed as described above. More specifically, theECU 10 sets the change rate when the engine speed is higher than thegiven speed N3 (i.e., the slope of the upper chart of FIG. 12) to behigher than the change rate when the engine speed is equal to or lowerthan the given speed N3 (i.e., in the example of the upper chart of FIG.12, the slope of the chart is zero).

Note that although not illustrated, when the engine speed is equal to orlower than the given speed N3, instead of setting the change rate of theinjection timing of the second-stage injection to zero, the injectiontiming may be advanced as the engine speed increases. In this case, thechange rate when the engine speed is equal to or lower than the givenspeed N3 may be lower than that when the engine speed exceeds the givenspeed N3.

As illustrated in the upper chart of FIG. 12, an upper limit value isalso determined for the injection timing of second-stage injection. Ifthe injection timing of the second-stage injection is excessively early,the flow in the combustion chamber 17 at the ignition timing becomesweak, which makes the SI combustion slow. When the SI combustion becomesslow, the self-ignition timing cannot accurately be controlled asdescribed above.

Therefore, the ECU 10 outputs a control signal to the injector 6 so asnot to exceed a given advance limit. Since the injection ratio of thesecond-stage injection is linearly advanced as the engine speedincreases, when the engine speed exceeds the given speed N4, the ECU 10outputs the control signal to the injector 6 so that the injectiontiming of the second-stage injection reaches the advance limit. As aresult, it is avoided that the SI combustion slows down, and thecontrollability of the self-ignition timing in the SPCCI combustion isprevented from degrading.

When the SI ratio increases due to the high engine load, since thecombustion noise is reduced, it is not required to advance thesecond-stage injection to increase the SI ratio. Therefore, the ECU 10may shift the speed N3 at which the injection timing starts to beadvanced, to the higher speed side, as illustrated by the one-dottedchain line in the waveform 121. In this manner, the range where theinjection timing of the second-stage injection is late is extended tothe higher speed side. Since the SI combustion in the SPCCI combustionbecomes sharp if the second-stage injection is late as described above,the controllability of the self-ignition timing improves.

Note that different from the example of the waveform 121, as indicatedby the one-dotted chain line in a waveform 122 of the lower chart ofFIG. 12, the ECU 10 may set the slope of the straight line indicatingthe relationship between the engine speed and the injection timing ofthe second-stage injection gentler.

In addition, although the ECU 10 changes both the second-stage injectionratio and the injection timing of the second-stage injection accordingto the change in the engine speed, the ECU 10 may alternatively changeonly the injection ratio of the second-stage injection according to thechange in the engine speed.

(Adjustment of SI Ratio)

FIG. 15 illustrates a flow regarding the operation control of the engineexecuted by the ECU 10. Based on the detection signals from the sensorsSW1 to SW16, the ECU 10 determines the operating state of the engine 1,and also adjusts the state function inside the combustion chamber 17,the injection amount, the injection timing, and the ignition timing sothat the combustion in the combustion chamber 17 is of the SI ratiocorresponding to the operating state. The ECU 10 further adjusts the SIratio when the adjustment thereof is determined to be needed, based onthe detection signals from the sensors.

First at 51, the ECU reads the detection signals of the sensors SW1 toSW16. Next at S2, the ECU 10 determines the operating state of theengine 1 based on the detection signals and sets the target SI ratiowhich is as illustrated in FIG. 9 or 14.

Next at S3, the ECU 10 sets a target in-cylinder state function forachieving the set target SI ratio, based on a preset combustion model.For example, the target temperature, the target pressure, and the targetstate function in the combustion chamber 17 are set. At S4, the ECU 10sets the opening of the EGR valve 54, the opening of the throttle valve43, the opening of the air bypass valve 48, the opening of the SCV 56,and the phase angles of the intake electrically-operated S-VT 23 and theexhaust electrically-operated S-VT 24 which are required for achievingthe target in-cylinder state function. The ECU 10 sets the controlamounts of these devices in advance based on a map stored in the ECU 10.Based on the set control amounts, the ECU 10 outputs control signals tothe EGR valve 54, the throttle valve 43, the air bypass valve 48, theSCV 56, the intake electrically-operated S-VT 23, and the exhaustelectrically-operated S-VT 24. As each device operates based on thecontrol signal from the ECU 10, the state function in the combustionchamber 17 becomes the target state function.

The ECU 10 further calculates a predicted value and an estimated valueof the state function in the combustion chamber 17 based on the setcontrol amount of each device. The state function predicted value isobtained by predicting the state function in the combustion chamber 17before the intake valve 21 is closed, and is used for setting the fuelinjection amount on the intake stroke as described later. The statefunction estimated value is obtained by estimating the state function inthe combustion chamber 17 after the intake valve 21 is closed, and isused for setting of the fuel injection amount and the ignition timing onthe compression stroke as described later. The state function estimatedvalue is also used for calculating a state function error based oncomparison with an actual combustion state as described later.

At S5, the ECU 10 sets the fuel injection amount on the intake strokebased on the state function predicted value. Note that in a case wherethe fuel injection is not performed on the intake stroke, the fuelinjection amount is zero. At S6, the ECU 10 controls the injection ofthe injector 6. That is, the ECU 10 outputs a control signal to theinjector 6 to inject the fuel into the combustion chamber 17 at a giveninjection timing.

At S7, the ECU 10 sets the fuel injection amount on the compressionstroke based on the state function estimated value and the fuelinjection result on the intake stroke. Note that in the case where thefuel injection is not performed on the compression stroke, the fuelinjection amount is zero. In the case where the split injections areperformed on the compression stroke, the injection amount of thefirst-stage injection and the injection amount of the second-stageinjection are respectively set. At S8, the ECU 10 outputs a controlsignal to the injector 6 to inject the fuel into the combustion chamber17 at the injection timing based on the preset map.

At S9, the ECU 10 sets the ignition timing based on the state functionestimated value and the fuel injection result on the compression stroke.At S10, the ECU 10 outputs a control signal to the spark plug 25 toignite the mixture gas into the combustion chamber 17 at the setignition timing.

The SI combustion or SPCCI combustion is performed in the combustionchamber 17 by the spark plug 25 igniting the mixture gas. At S11, theECU 10 reads a change in pressure inside the combustion chamber 17detected by the pressure sensor SW6, and, based on this change,determines the combustion state of the mixture gas in the combustionchamber 17. At S12, the ECU 10 further compares the detection result ofthe combustion state with the state function estimated value estimatedat S4 to calculate a difference (error) between the state functionestimated value and the actual state function. The calculated error isused in the estimation at S4 in the next cycle and/or its subsequentcycles. The ECU 10 adjusts the openings of the throttle valve 43, theEGR valve 54, the SCV 56, and/or the air bypass valve 48, and the phaseangles of the intake electrically-operated S-VT 23 and the exhaustelectrically-operated S-VT 24. Thus, the introduction amounts of freshair and EGR gas amount into the combustion chamber 17 are adjusted. Thefeedback of the state function error is equivalent to adjusting the SIratio when the ECU 10 determines that the adjustment of the SI ratio isneeded based on the error between the target SI ratio and the actual SIratio (error).

If, at S8, the temperature in the combustion chamber 17 is estimated tobe lower than the target temperature based on the state functionestimated value, the ECU 10 advances the injection timing on thecompression stroke than the injection timing which is based on the mapso that the ignition timing advances. On the other hand, if thetemperature in the combustion chamber 17 is estimated to be higher thanthe target temperature based on the state function estimated value atS8, the ECU 10 retards the injection timing on the compression strokethan the injection timing which is based on the map so that the ignitiontiming retards.

That is, as indicated by P2 of FIG. 16, if the temperature in thecombustion chamber 17 is low, the timing θ_(CI) at which the unburnedmixture gas self-ignites after the SI combustion is started by thespark-ignition retards, and the SI ratio shifts from the target SI ratio(see P1). In this case, the unburned fuel increases and the exhaust gasperformance degrades.

Therefore, when the temperature in the combustion chamber 17 isestimated to be lower than the target temperature, the ECU 10 advancesthe injection timing as well as the ignition timing θ_(IG) at S10 ofFIG. 15. As indicated by P3 in FIG. 16, since the start of SI combustionis advanced, sufficient heat generation by the SI combustion occurs, andtherefore, the timing θ_(CI) of the self-ignition of the unburnedmixture gas is prevented from retarding when the temperature in thecombustion chamber 17 is low. As a result, the SI ratio approaches thetarget SI ratio. The increase of the unburned fuel and the degradationof exhaust gas performance are prevented.

Moreover, as indicated by P4 of FIG. 16, when the temperature in thecombustion chamber 17 is high, the unburned mixture gas self-ignitesimmediately after the SI combustion is started by the spark-ignition,and the SI ratio shifts from the target SI ratio (see P1). In this case,the combustion noise increases.

Therefore, when the temperature in the combustion chamber 17 isestimated to be exceed the target temperature, the ECU 10 retards theinjection timing as well as the ignition timing θIG at S10 of FIG. 15.As indicated by P5 in FIG. 16, since the start of SI combustion isretarded, the timing θ_(CI) of the self-ignition of the unburned mixturegas is prevented from advancing when the temperature in the combustionchamber 17 is high. As a result, the SI ratio approaches the target SIratio. The increase of combustion noise is avoided.

The adjustments of the injection timing and the ignition timing areequivalent to adjusting the SI ratio when the ECU 10 determines that theSI ratio in the SPCCI combustion is required to be adjusted. Byadjusting the injection timing, suitable mixture gas is formed in thecombustion chamber 17 at the advanced or retarded ignition timing. Thespark plug 25 reliably ignites the mixture gas, and the unburned mixturegas self-ignites at a suitable timing.

Note that the adjustment of the state function inside the combustionchamber 17 through controlling the throttle valve 43, the EGR valve 54,the air bypass valve 48, the intake electrically-operated S-VT 23, theexhaust electrically-operated S-VT 24, and the SCV 56 based on theactual combustion state in FIG. 16 is as described at S12 and S4 of FIG.15.

The engine 1 adjusts the SI ratio by the state function setting devicesincluding the throttle valve 43, the EGR valve 54, the air bypass valve48, the intake electrically-operated S-VT 23, the exhaustelectrically-operated S-VT 24, and the SCV 56. The SI ratio may roughlybe adjusted by adjusting the state function inside the combustionchamber 17. At the same time, the engine 1 adjusts the SI ratio byadjusting the injection timing and the ignition timing of the fuel. Byadjusting the injection timing and the ignition timing, for example, thedifference between the cylinders may be corrected and the self-ignitiontiming is finely adjusted. By adjusting the SI ratio in two stages, theengine 1 accurately achieves the target SPCCI combustion correspondingto the operating state.

(Second Configuration Example of Engine Operating Range Map)

FIG. 7B illustrates a second configuration example of the operatingrange map of the engine 1. An operating range map 701 is divided intothree ranges in terms of the engine load. For example, the three rangesinclude a low load range (A) including the idle operation, a high loadrange (C) including the full engine load, a medium load range (B)between the low load range (A) and the high load range (C). The low loadrange (A) of the operating range map 701 corresponds to the low loadrange (A) of the operating range map 700 of FIG. 7A, the medium loadrange (B) of the operating range map 701 corresponds to the medium loadrange (B) of the operating range map 700 of FIG. 7A, and the high loadrange (C) of the operating range map 701 corresponds to the high loadrange (C) of the operating range map 700 of FIG. 7A. The medium loadrange (B) is divided into a first medium load segment (B1) and a secondmedium load segment (B2) in the engine load direction, and the high loadrange (C) is divided into a first high load segment (C1) and a secondhigh load segment (C2) in the engine speed direction.

On the operating range map 701 of FIG. 7B, no high speed range (D) ofthe operating range map 700 is provided, and the low load range (A), themedium load range (B) and the high load range (C) are respectivelyextended to a highest speed N2. Note that a speed N1 of FIG. 7Bcorresponds to the speed N1 of FIG. 7A in terms of being the lowestengine speed for performing the SPCCI combustion, and the speed N2 ofFIG. 7B corresponds to the speed N2 of FIG. 7A in terms of being thehighest engine speed for performing the SPCCI combustion.

Also on the operating range map 701 of FIG. 7B, the control in theengine load direction is performed as described with reference to FIGS.9 and 10, and, within the medium load range (B) in which the SPCCIcombustion is performed, the SI ratio in the SPCCI combustion may bechanged by changing the second-stage injection ratio and/or thesecond-stage injection timing in the engine speed direction as describedwith reference to FIGS. 11 to 14.

(Third Configuration Example of Engine Operating Range Map)

FIG. 7C illustrates another configuration example of the operating rangemap of the engine 1 in a warmed-up state of the engine. An operatingrange map 702 of the engine 1 is divided into five ranges in terms ofthe engine load and the engine speed. For example, the five rangesinclude: a low load range (1)-1 including the idle operation andextending in low and medium engine speed ranges; a medium load range(1)-2 in which the engine load is higher than the low load range andextending in the low and medium engine speed ranges; a high-loadmedium-speed range (2) in which the engine load is higher than themedium load range (1)-2 and which is located in the high load rangeincluding the full engine load; a high-load low-speed range (3) locatedin the high load range and in which the engine speed is lower than thehigh-load medium-speed range (2); and a high speed range (4) in whichthe engine speed is higher than the low load range (1)-1, the mediumload range (1)-2, the high-load medium-speed range (2), and thehigh-load low-speed range (3). Here, the low speed range, the mediumspeed range, and the high speed range may be defined by substantiallyevenly dividing, in the engine speed direction, the entire operatingrange of the engine 1 into three ranges of the low speed range, themedium speed range and the high speed range. In the example of FIG. 7C,the engine speed lower than a speed N5 is defined as low, the enginespeed higher than a speed N2 is defined as high, and the engine speedbetween engine speeds N5 and N2 is defined as medium. For example, thespeed N5 may be about 1,200 rpm and the speed N2 may be about 4,000 rpm.Note that the speed N2 in FIG. 7C corresponds to the speed N2 of FIGS.7A and 7B in terms of being the highest engine speed for performing theSPCCI combustion. Further, the two-dotted chain line in FIG. 7Cindicates the road-load line of the engine 1.

Here, in the third configuration example, the geometric compressionratio of the engine 1 is set relatively low. Reducing the geometriccompression ratio is advantageous in reducing a cooling loss and amechanical loss. For example, the geometric compression ratio may be setto 14:1 to 17:1 in regular specifications (the octane number of the fuelis about 91) and to 15:1 to 18:1 in high-octane specifications (theoctane number of the fuel is about 96).

While reducing the geometric compression ratio, in order to increase thetemperature inside the combustion chamber 17 to a certain extent whenthe piston 3 reaches CTDC, in the third configuration example, theeffective compression ratio is increased. That is, the close timing ofthe intake valve 21 is advanced to approach BDC of the intake stroke.Even if the internal EGR gas is to be introduced into the combustionchamber 17 by providing the negative overlap period in which both theintake valve 21 and the exhaust valve 22 are closed as described above,since the open period of the intake valve 21 needs to be set to theadvancing side in order to increase the effective compression ratio, theintroduction amount of the internal EGR gas cannot be increased.Therefore, in the third configuration example, the internal EGR gas isintroduced into the combustion chamber 17 by providing the positiveoverlap period in which the intake and exhaust valves 21 and 22 are bothopened. Thus, both of increasing the effective compression ratio andintroducing the internal EGR gas into the combustion chamber 17 areachieved.

On the operating range map 702, mainly for improving the fuel efficiencyand the exhaust gas performance, the engine 1 performs combustion bycompression self-ignition (i.e., SPCCI combustion) within the low loadrange (1)-1, the medium load range (1)-2, and the high-load medium-speedrange (2). The operating range map 702 is different from the operatingrange map 700 or 701 in that the SPCCI combustion is performed also whenthe engine 1 operates at a low load and when the engine 1 operates at ahigh load. Further, the engine 1 performs the combustion byspark-ignition within the other ranges, specifically, the high-loadlow-speed range (3) and the high speed range (4).

In the third configuration example, a swirl flow is formed in thecombustion chamber 17 by setting the opening of the SCV 56 to theclosing side at least within the range in which the SPCCI combustion isperformed.

According to the study of the present inventors, by generating the swirlflow in the combustion chamber 17, the residual gas (i.e., burned gas)accumulated in the cavity 31 on the top surface of the piston 3 is ledoutside the cavity 31. When the fuel is distributed substantially evenlythroughout the combustion chamber 17, the G/F of the mixture gas nearthe spark plug 25 is relatively lowered since no residual gas is locatedin the cavity 31, the G/F of the mixture gas located away from the sparkplug 25 is relatively increased since it includes the residual gas. TheG/F of the mixture gas in the combustion chamber 17 may be stratified.

The SI combustion in the SPCCI combustion is the combustion of themixture gas ignited by the spark plug 25. The mixture gas near the sparkplug 25 mainly combusts in the SI combustion. On the other hand, the CIcombustion in the SPCCI combustion is the combustion caused byself-ignition of the unburned mixture after the SI combustion starts.The mixture gas located away from the spark plug 25 mainly combusts inthe CI combustion.

When the G/F in the combustion chamber 17 is stratified, the upper limitvalue of the total G/F in the entire combustion chamber 17 forstabilizing the SPCCI combustion exceeds the range of 18 to 30 describedabove. According to the study of the present inventors, the SPCCIcombustion can be stabilized if the total G/F range is from 18 to 50.Here, the range of G/F of the mixture gas near the spark plug 25 is from14 to 22. Stratifying the G/F of the mixture gas in the combustionchamber 17 further dilutes the mixture gas while stabilizing the SPCCIcombustion, which is advantageous in improving the fuel efficiency ofthe engine.

Hereinafter, the operation of the engine 1 within each range of theoperating range map 702 is described in detail with reference to thefuel injection timing and the ignition timing illustrated in FIG. 17.Note that the reference characters 601, 602, 603, 604, 605, and 606 inFIG. 17 correspond to the operating state of the engine 1 indicated bythe reference characters 601, 602, 603, 604, 605, and 606 on theoperating range map 702 of FIG. 7C.

(Low Load Range (1)-1)

When the engine 1 is operating within the low load range (1)-1, theengine 1 performs the SPCCI combustion.

The reference character 601 in FIG. 17 indicates one example of the fuelinjection timings (reference characters 6011 and 6012) and the ignitiontiming (reference character 6013), and a combustion waveform (i.e., awaveform indicating a change in heat generation rate with respect to thecrank angle; the reference character 6014) when the engine 1 isoperating in the operating state of the reference character 601 withinthe low load range (1)-1 of the engine 1.

In order to improve the fuel efficiency of the engine 1, the EGR system55 introduces the EGR gas into the combustion chamber 17 when the engine1 is operating within the low load range (1)-1. For example, byproviding the positive overlap period in which the intake valve 21 andthe exhaust valve 22 are both opened near TDC of the exhaust stroke, aportion of the exhaust gas discharged from the combustion chamber 17 tothe intake port 18 and the exhaust port 19 is reintroduced into thecombustion chamber 17. Since hot burned gas is introduced into thecombustion chamber 17 in this case, the temperature in the combustionchamber 17 increases, which is advantageous in stabilizing the SPCCIcombustion.

When the engine 1 is operating within the low load range (1)-1, theswirl flow is formed in the combustion chamber 17. The swirl flow isstrong in an outer circumferential portion of the combustion chamber 17and weak in a center portion. The SCV 56 is fully closed or has a givennarrow opening. As described above, since the intake port 18 is a tumbleport, an oblique swirl flow having a tumble component and a swirlcomponent is formed in the combustion chamber 17.

When the engine 1 operates within the low load range (1)-1, the air-fuelratio (A/F) of the mixture gas is leaner than the stoichiometricair-fuel ratio in the entire combustion chamber 17. That is, the excessair ratio λ of the mixture gas exceeds 1 in the entire combustionchamber 17. More specifically, the A/F of the mixture gas in the entirecombustion chamber 17 is 30:1 or higher. In this manner, generation ofraw NO_(x) is reduced and the exhaust gas performance is improved.

When the engine 1 operates within the low load range (1)-1, the mixturegas is stratified between the center portion and the outercircumferential portion of the combustion chamber 17. The center portionof the combustion chamber 17 is the portion where an ignition plug 25 isdisposed, and the outer circumferential portion is the portion aroundthe center portion and in contact with a liner of the cylinder 11. Thecenter portion of the combustion chamber 17 may be defined as a portionwhere the swirl flow is weak and the outer circumferential portion maybe defined as a portion where the swirl flow is strong.

The fuel concentration of the mixture gas in the center portion ishigher than that in the outer circumferential portion. For example, theA/F of the mixture gas in the center portion is between 20:1 and 30:1,and the A/F of the mixture gas in the outer circumferential portion is35:1 or higher.

When the engine 1 operates within the low load range (1)-1, the injector6 basically injects the fuel into the combustion chamber 17 by splittingit into in a plurality of injections on the compression stroke. Themixture gas is stratified in the center portion and the outercircumferential portion of the combustion chamber 17 by the splitinjections of the fuel and the strong swirl flow in the combustionchamber 17.

After the fuel injection is ended, the spark plug 25 ignites the mixturegas in the center portion of the combustion chamber 17 at the giventiming before CTDC (see the reference character 6013). Since the fuelconcentration of the mixture gas in the center portion is relativelyhigh, ignitability improves and the SI combustion by the flamepropagation stabilizes. By stabilizing the SI combustion, the CIcombustion starts at the suitable timing. The controllability of the CIcombustion improves in the SPCCI combustion. As a result, when theengine 1 operates within the low load range (1)-1, both the reduction ofthe generation of combustion noise and the improvement of the fuelefficiency by the shortening of the compression period are achieved.

Since the engine 1 performs the SPCCI combustion by making the mixturegas leaner than the stoichiometric air-fuel ratio within the low loadrange (1)-1 as described above, the low load range (1)-1 may be referredto as “SPCCI lean range.”

(Medium Load Range (1)-2)

Also when the engine 1 is operating within the medium load range (1)-2,the engine 1 performs the SPCCI combustion similarly to the low loadrange (1)-1. The medium load range (1)-2 corresponds to the medium loadrange (B) on the operating range map 700 or 701.

The reference character 602 in FIG. 17 indicates one example of the fuelinjection timings (reference characters 6021 and 6022) and the ignitiontiming (reference character 6023), and a combustion waveform (referencecharacter 6024) when the engine 1 is operating in the operating state ofthe reference character 602 within the medium load range (1)-2 of theengine 1.

The EGR system 55 introduces the EGR gas into the combustion chamber 17when the operating state of the engine 1 is within the medium load range(1)-2. For example, similar to the low load range (1)-1, by providingthe positive overlap period in which the intake valve 21 and the exhaustvalve 22 are both opened near TDC of the exhaust stroke, a portion ofthe exhaust gas discharged from the combustion chamber 17 to the intakeport 18 and the exhaust port 19 is reintroduced into the combustionchamber 17. That is, the internal EGR gas is introduced into thecombustion chamber 17. Further, within the medium load range (1)-2, theexhaust gas cooled by the EGR cooler 53 is introduced into thecombustion chamber 17 through the EGR passage 52. That is, the externalEGR gas at a lower temperature than the internal EGR gas is introducedinto the combustion chamber 17. Within the medium load range (1)-2, theinternal EGR gas and/or external EGR gas is introduced into thecombustion chamber 17 to adjust the temperature in the combustionchamber 17 to an appropriate temperature.

Also when the engine 1 operates within the medium load range (1)-2,similar to the low load range (1)-1, a swirl flow is formed in thecombustion chamber 17. The SCV 56 is fully closed or has a given narrowopening. By forming the swirl flow, the residual gas accumulated in thecavity 31 is led outside the cavity 31. As a result, the G/F of themixture gas in the SI portion near the spark plug 25 may be varied fromthe G/F of the mixture gas in the CI portion around the SI portion.Therefore, the SPCCI combustion is stabilized by setting the total G/Fin the entire combustion chamber 17 to between 18 and 50 as describedabove.

Further, since the turbulence kinetic energy in the combustion chamber17 increases by forming the swirl flow, when the engine 1 operateswithin the medium load range (1)-2, the flame of the SI combustionpropagates promptly and the SI combustion is stabilized. Thecontrollability of the CI combustion improves by stabilizing the SIcombustion. By making the timing of the CI combustion in the SPCCIcombustion appropriate, the generation of combustion noise is reducedand the fuel efficiency is improved. Further, the variation in torquebetween cycles is reduced.

When the engine 1 operates within the medium load range (1)-2, theair-fuel ratio (A/F) of the mixture gas is at the stoichiometricair-fuel ratio (A/F≈4.7:1) in the entire combustion chamber 17. Thethree-way catalyst purifies the exhaust gas discharged from thecombustion chamber 17. Thus, the exhaust gas performance of the engine 1becomes good. The A/F of the mixture gas may be set to remain within thepurification window of the three-way catalyst. Therefore, the excess airratio λ of the mixture gas may be set to 1.0±0.2.

When the engine 1 operates within the medium load range (1)-2, theinjector 6 performs the fuel injection on the intake stroke (referencecharacter 6021) and the fuel injection on the compression stroke(reference character 6022). By performing the first injection 6021 onthe intake stroke, the fuel is distributed substantially evenly in thecombustion chamber 17. By performing the second injection 6022 on thecompression stroke, the temperature in the combustion chamber 17 isreduced by latent heat of vaporization of the fuel. The mixture gascontaining the fuel injected in the first injection 6021 is preventedfrom causing pre-ignition. Note that within the medium load range (1)-2,particularly in the operating state where the engine load is low, thesecond injection 6022 may be omitted.

When the injector 6 performs the first injection 6021 on the intakestroke and the second injection 6022 on the compression stroke, themixture gas with the excess air ratio λ of 1.0±0.2 as a whole is formedin the combustion chamber 17. Since the fuel concentration of themixture gas is substantially homogeneous, the improvement in the fuelefficiency by reducing the unburned fuel loss and the improvement in theexhaust gas performance by avoiding the smoke generation are achieved.The excess air ratio λ is preferably 1.0 to 1.2. Further, the total G/Fin the entire combustion chamber 17 is 18 to 50, and the G/F of the SIportion near the spark plug 25 is 14 to 22.

By the spark plug 25 igniting the mixture gas at the given timing beforeCTDC (reference character 6023), the mixture gas combusts by flamepropagation. After the combustion by flame propagation is started, theunburned mixture gas self-ignites at the target timing and causes the CIcombustion. The fuel injected in the second-stage injection mainlycauses the SI combustion. The fuel injected in the first-stage injectionmainly causes the CI combustion. By having the total G/F in the entirecombustion chamber 17 between 18 and 50 and the G/F of the SI portionnear the spark plug 25 between 14 and 22, the SPCCI combustion isstabilized.

Therefore, within the medium load range (1)-2, since the engine 1performs the SPCCI combustion by setting the mixture gas to thestoichiometric air-fuel ratio, the medium load range (1)-2 may bereferred to as “SPCCIλ=1 range.”

Here, on the operating range map 702, a range in which the booster 44 isturned off (refer to S/C OFF) is a part of the low load range (1)-1 anda part of the medium load range (1)-2. In detail, the booster 44 isturned off within the low engine speed segment of the low load range(1)-1. Within a high engine speed segment of the low load range (1)-1,the booster 44 is turned on to increase the boosting pressure in orderto secure a required intake charge amount corresponding to the enginespeed being high. Further, within a low-load low-speed segment of themedium load range (1)-2, the booster 44 is turned off. Within a highload segment of the medium load range (1)-2, the booster 44 is turned onin order to secure the required intake charge amount corresponding tothe engine speed being high. Within a high speed segment of the mediumload range (1)-2, the booster 44 is turned on in order to secure therequired intake charge amount corresponding to the engine speed beinghigh.

Note that within the high-load medium-speed range (2), the high-loadlow-speed range (3), and the high speed range (4), the booster 44 isturned on throughout the ranges.

(High-Load Medium-Speed Range (2))

Also when the engine 1 is operating within the high-load medium-speedrange (2), the engine 1 performs the SPCCI combustion similarly to thelow load range (1)-1 and the medium load range (1)-2.

The reference character 603 in FIG. 17 indicates one example of the fuelinjection timings (reference characters 6031 and 6032) and the ignitiontiming (reference character 6033), and a combustion waveform (referencecharacter 6034) when the engine 1 is operating in the operating state ofthe reference character 603 within the high-load medium-speed range (2)of the engine 1. Further, the reference character 604 in FIG. 17indicates one example of the fuel injection timings (reference character6041) and the ignition timing (reference character 6042), and acombustion waveform (reference character 6043) when the engine speed ishigher than in the operating state of the reference character 603.

The EGR system 55 introduces the EGR gas into the combustion chamber 17when the operating state of the engine 1 is within the high-loadmedium-speed range (2). The engine 1 reduces the EGR gas amount as theengine load increases. At the full engine load, the EGR gas may be setto zero.

Also when the engine 1 operates within the high-load medium-speed range(2), similar to the low load range (1)-1, a swirl flow is formed in thecombustion chamber 17. The swirl flow may be, for example, a strongswirl flow at a swirl ratio of 4 or higher. The SCV 56 is fully closedor has a given narrow opening.

When the engine 1 operates within the high-load medium-speed range (2),the air-fuel ratio (A/F) of the mixture gas is at or richer than thestoichiometric air-fuel ratio in the entire combustion chamber 17 (i.e.,the excess air ratio λ of the mixture gas is λ≤1).

When the engine 1 operates at the low speed side of the high-loadmedium-speed range (2), the injector 6 injects the fuel on the intakestroke (reference character 6031) and injects the fuel at a final stageof the compression stroke (reference character 6032). The final stage ofthe compression stroke may be defined by evenly dividing the compressionstroke into three stages of an initial stage, an intermediate stage, andthe final stage.

The first-stage injection 6031 starting on the intake stroke may startthe fuel injection in an early half of the intake stroke. The early halfof the intake stroke may be defined by evenly dividing the intake strokeinto two parts of the early half and the latter half. For example, thefirst-stage injection may start the fuel injection at 280° CA beforeTDC.

When the injection of the first-stage injection 6031 is started in theearly half of the intake stroke, although not illustrated, the fuelspray hits an opening edge of the cavity 31 so that a portion of thefuel enters the squish area 171 of the combustion chamber 17 and therest of the fuel enters the section within the cavity 31. The swirl flowis strong in the outer circumferential portion of the combustion chamber17 and weak in the center portion. Therefore, the portion of the fuelthat enters the squish area 171 joins the swirl flow, and the remainingfuel that enters the section within the cavity 31 joins the inner sideof the swirl flow. The fuel joins the swirl flow, remains in the swirlflow during the intake stroke and the compression stroke and forms themixture gas for the CI combustion in the outer circumference portion ofthe combustion chamber 17. The fuel that enters the inner side of theswirl flow also remains at the inner side of the swirl flow during theintake stroke and the compression stroke and forms the mixture gas forthe SI combustion in the center portion of the combustion chamber 17.

When the engine 1 operates within the high-load medium-speed range (2),the excess air ratio λ of the mixture gas in the center portion wherethe ignition plug 25 is disposed preferably is 1 or less, and the excessair ratio λ of the mixture gas in the outer circumferential portion is 1or less, preferably below 1. The air-fuel ratio (A/F) of the mixture gasin the center portion may be, for example, between 13 and thestoichiometric air-fuel ratio (14.7:1). The air-fuel ratio of themixture gas in the center portion may be leaner than the stoichiometricair-fuel ratio. Further, the air-fuel ratio of the mixture gas in theouter circumferential portion may be, for example, between 11:1 and thestoichiometric air-fuel ratio, preferably between 11:1 and 12:1. Sincethe amount of fuel within the mixture gas increases in the outercircumferential portion when the excess air ratio λ of the outercircumferential portion of the combustion chamber 17 is set to below 1,the temperature is lowered by the latent heat of vaporization of thefuel. The air-fuel ratio of the mixture gas in the entire combustionchamber 17 may be between 12.5:1 and the stoichiometric air-fuel ratio,preferably between 12.5:1 and 13:1.

The second-stage injection 6032 performed in the final stage of thecompression stroke may start the fuel injection at 10° CA before TDC. Byperforming the second-stage injection immediately before TDC, thetemperature in the combustion chamber 17 is lowered by the latent heatof vaporization of the fuel. Although a low-temperature oxidationreaction of the fuel injected by the first-stage injection 6031progresses on the compression stroke and transitions to ahigh-temperature oxidation reaction before TDC, by performing thesecond-stage injection 6032 immediately before TDC so as to lower thetemperature inside the compression stroke, the transition from thelow-temperature oxidation reaction to the high-temperature oxidationreaction is avoided and pre-ignition is prevented. Note that the ratiobetween the injection amount of the first-stage injection and theinjection amount of the second-stage injection may be, for example,95:5.

The spark plug 25 ignites the mixture gas in the center portion ofcombustion chamber 17 near CTDC (reference character 6033). The sparkplug 25 ignites, for example, after CTDC. Since the spark plug 25 isdisposed in the center portion of the combustion chamber 17, theignition of the ignition plug 25 causes the mixture gas in the centerportion to start the SI combustion by flame propagation.

Within the high load range, the fuel injection amount increases as wellas the temperature of the combustion chamber 17, therefore the CIcombustion is likely to start early. In other words, within the highload range, the pre-ignition of the mixture gas is likely to occur.However, since the temperature of the outer circumferential portion ofthe combustion chamber 17 is lowered by the latent heat of vaporizationof the fuel as described above, the CI combustion is avoided fromstarting immediately after the mixture gas is spark-ignited.

As described above, when the spark plug 25 ignites the mixture gas inthe center portion, the combustion speed increases and the SI combustionis stabilized by high turbulence kinetic energy, and the flame of the SIcombustion propagates in the circumferential direction along the strongswirl flow inside the combustion chamber 17. Then, at a given positionof the outer circumferential portion of the combustion chamber 17 in thecircumferential direction, the unburned mixture gas ignites by beingcompressed and the CI combustion starts.

In the concept of this SPCCI combustion, by the combination ofstratifying the mixture gas in the combustion chamber 17 and causing thestrong swirl flow in the combustion chamber 17, the SI combustion issufficiently performed until the CI combustion starts. As a result, thegeneration of combustion noise is reduced, and since the combustiontemperature does not become excessively high, generation of NO_(x) isalso reduced. Further, the variation in torque between cycles isreduced.

Further, since the temperature in the outer circumferential portion islow, the CI combustion becomes slower and the generation of thecombustion noise is reduced. Moreover, since the combustion period isshortened by the CI combustion, within the high load range, the torqueimproves and also the thermal efficiency improves. Thus, by performingthe SPCCI combustion within the high engine load range, the engine 1 isimproved in the fuel efficiency while avoiding the combustion noise.

When the engine 1 operates at the high speed side of the high-loadmedium-speed range (2), the injector 6 starts the fuel injection on theintake stroke (reference character 6041).

The first-stage injection 6041 starting on the intake stroke may startthe fuel injection in the early half of the intake stroke similarly tothe above description. For example, the first-stage injection 6041 maystart the fuel injection at 280° CA before TDC. The first-stageinjection may last over the intake stroke and end on the compressionstroke. By setting the start of injection of the first-stage injection6041 in the early half of the intake stroke, the mixture gas for the CIcombustion is formed in the outer circumferential portion of thecombustion chamber 17 and the mixture gas for the SI combustion isformed in the center portion of the combustion chamber 17. Similar tothe above description, the excess air ratio λ of the mixture gas in thecenter portion where the ignition plug 25 is disposed preferably is 1 orless, and the excess air ratio λ of the mixture gas in the outercircumferential portion is 1 or less, preferably below 1. The air-fuelratio (A/F) of the mixture gas in the center portion may be, forexample, between 13 and the stoichiometric air-fuel ratio (14.7:1). Theair-fuel ratio of the mixture gas in the center portion may be leanerthan the stoichiometric air-fuel ratio. Further, the air-fuel ratio ofthe mixture gas in the outer circumferential portion may be, forexample, between 11:1 and the stoichiometric air-fuel ratio, preferablybetween 11:1 and 12:1. The air-fuel ratio of the mixture gas in theentire combustion chamber 17 may be between 12.5:1 and thestoichiometric air-fuel ratio, preferably between 12.5:1 and 13:1.

When the engine speed increases, the time length during which the fuelinjected in the first-stage injection 6041 reacts becomes shorter.Therefore, the second-stage injection for suppressing the oxidationreaction of the mixture gas may be omitted.

The spark plug 25 ignites the mixture gas in the center portion ofcombustion chamber 17 near CTDC (reference character 6042). The sparkplug 25 ignites, for example, after CTDC.

As described above, by stratifying the mixture gas, within the high-loadmedium-speed range (2), the combustion noise is reduced and the SPCCIcombustion is stabilized.

Since the engine 1 performs the SPCCI combustion by setting the mixturegas to or leaner than the stoichiometric air-fuel ratio within thehigh-load medium-speed range (2) as described above, the high-loadmedium-speed range (2) may be referred to as “SPCCIλ≤1 range.”

(High-Load Low-Speed Range (3))

When the engine 1 is operating within the high-load low-speed range (3),the engine 1 performs the SI combustion instead of the SPCCI combustion.The high-load low-speed range (3) corresponds to the first high loadsegment (C1) on the operating range map 700 or 701.

The reference character 605 in FIG. 17 indicates one example of the fuelinjection timings (reference characters 6051 and 6052) and the ignitiontiming (reference character 6053), and a combustion waveform (referencecharacter 6054) when the engine 1 is operating in the operating state ofthe reference character 605 within the high-load low-speed range (3) ofthe engine 1.

The EGR system 55 introduces the EGR gas into the combustion chamber 17when the operating state of the engine 1 is within the high-loadlow-speed range (3). The engine 1 reduces the EGR gas amount as theengine load increases. At the full load, the EGR gas may be set to zero.

When the engine 1 operates within the high-load low-speed range (3), theair-fuel ratio (A/F) of the mixture gas is at the stoichiometricair-fuel ratio (A/F≈44.7:1) in the entire combustion chamber 17. The A/Fof the mixture gas may be set to remain within the purification windowof the three-way catalyst. Therefore, the excess air ratio λ of themixture gas may be set to 1.0±0.2. By setting the air-fuel ratio of themixture gas to the stoichiometric air-fuel ratio, the fuel efficiencyimproves within the high-load low-speed range (3). When the engine 1operates within the high-load low-speed range (3), the fuelconcentration of the mixture gas in the entire combustion chamber 17 maybe set so that the excess air ratio λ is 1 or less and equal to orhigher than the excess air ratio λ within the high-load medium-speedrange (2), preferably higher than the excess air ratio λ within thehigh-load medium-speed range (2).

On the operating range map 702, when the engine 1 operates within thehigh-load low-speed range (3), the injector 6 injects the fuel into thecombustion chamber 17 at the timings of the intake stroke and in theretard period from the final stage of the compression stroke to theearly stage of the expansion stroke (reference characters 6051 and6052). By injecting the fuel in two injections, the amount of fuelinjected in the retard period is reduced. By injecting the fuel on theintake stroke (reference character 6051), the formation period of timeof the mixture gas is sufficiently secured. Additionally, by injectingthe fuel in the retard period (reference character 6052), the flow inthe combustion chamber 17 immediately before the ignition isstrengthened, which is advantageous in stabilizing the SI combustion.

After the fuel is injected, the spark plug 25 ignites the mixture gas ata timing near CTDC (reference character 6053). The spark plug 25ignites, for example, after CTDC. The mixture gas causes the SIcombustion on the expansion stroke. Since the SI combustion starts onthe expansion stroke, the CI combustion does not start.

In order to avoid the pre-ignition, the injector 6 may retard the fuelinjection timing as the engine speed decreases. The fuel injection inthe retard period may end on the expansion stroke.

When the engine 1 operates within the high-load low-speed range (3), theswirl flow is made weaker than when operating within the high-loadmedium-speed range (2). When the engine 1 operates within the high-loadlow-speed range (3), the opening of the SCV 56 is larger than whenoperating within the high-load medium-speed range (2). The opening ofthe SCV 56 may be, for example, about 50% (i.e., half opened).

As indicated by the one-dotted chain line in the upper chart of FIG. 2,the axes of the nozzle ports of the injector 6 do not circumferentiallyoverlap with the spark plug 25. The fuel injected from the nozzle portsflows in the circumferential direction due to the swirl flow in thecombustion chamber 17. By the swirl flow, the fuel is promptly conveyedto near the spark plug 25. The fuel is vaporized while being conveyed tonear the spark plug 25.

On the other hand, if the swirl flow is excessively strong, the fuelflows in the circumferential direction and reaches away from the sparkplug 25, and the fuel cannot promptly be conveyed to near the spark plug25. For this reason, when the engine 1 operates within the high-loadlow-speed range (3), the swirl flow is made weaker than when operatingwithin the high-load medium-speed range (2). As a result, the fuel ispromptly conveyed to near the spark plug 25, thus the ignitability ofthe mixture gas improves and the SI combustion stabilizes.

Within the high-load low-speed range (3), since the engine 1 performsthe SI combustion by injecting the fuel in the retard period from thefinal stage of the compression stroke to the early stage of theexpansion stroke, the high-load low-speed range (3) may be referred toas “retarded SI range.”

(High Speed Range (4))

When the engine speed is high, the time length for the crank angle tochange 1° becomes shorter. Therefore, for example, within the high speedside of the high load range, it is difficult to stratify the mixture gasin the combustion chamber 17 as described above. When the engine speedincreases, it becomes difficult to perform the SPCCI combustiondescribed above.

When the engine 1 is operating within the high speed range (4), theengine 1 performs the SI combustion instead of the SPCCI combustion.Note that the high speed range (4) extends over the entire loaddirection from low load to high loads.

The reference character 606 in FIG. 17 indicates one example of the fuelinjection timing (reference characters 6061) and the ignition timing(reference character 6062), and a combustion waveform (referencecharacter 6063) when the engine 1 is operating in the operating state ofthe reference character 606 within the high speed range (4) of theengine 1.

The EGR system 55 introduces the EGR gas into the combustion chamber 17when the operating state of the engine 1 is within the high speed range(4). The engine 1 reduces the EGR gas amount as the engine loadincreases. At the full load, the EGR gas may be set to zero.

When operating in the high speed range (4), the engine 1 fully opens theSCV 56. No swirl flow is generated in the combustion chamber 17, andonly the tumble flow is generated. By fully opening the SCV 56, thecharging efficiency is improved in the high speed range (4) and apumping loss is reduced.

When the engine 1 operates within the high speed range (4), the air-fuelratio (A/F) of the mixture gas is basically at the stoichiometricair-fuel ratio (A/F≈14.7:1) in the entire combustion chamber 17. Theexcess air ratio λ of the mixture gas may be set to 1.0±0.2. Note thatwithin the high load side of the high speed range (4) including the fullload, the excess air ratio λ of the mixture gas may be less than 1.

When the engine 1 operates within the high speed range (4), the injector6 starts the fuel injection on the intake stroke (reference character6061). The injector 6 injects all the fuel for one combustion cycle in alump. By starting the fuel injection on the intake stroke, homogeneousor substantially homogeneous mixture gas is formed in the combustionchamber 17. Further, when the engine speed is high, since thevaporization time of the fuel is secured as long as possible, theunburned fuel loss and generation of soot are reduced.

After the fuel injection is ended, the spark plug 25 ignites the mixturegas at a suitable timing before CTDC (reference character 6062).

Therefore, within the high speed range (4), since the engine 1 startsthe fuel injection on the intake stroke and performs the SI combustion,the high speed range (4) may be referred to as “intake SI range.”

(Operation Control of Engine in Engine Speed Direction in ThirdConfiguration Example of Operating Range Map)

A waveform 132 in the lower part of FIG. 13 illustrates a relationshipbetween the engine speed and the opening of the SCV 56 within the rangewhere the SPCCI combustion is performed (particularly, the SPCCIλ>1range and the SPCCIλ=1 range) on the operating range map 702 of FIG. 7C.The two-dotted chain line in the lower part of FIG. 13 indicates awaveform 131 in the upper chart of FIG. 13. As described above, withinthe SPCCI range of the operating range map 702, the opening of the SCV56 is set to the closing side regardless of the engine speed. Thewaveform 132 is different in this regard from the waveform 131 in whichthe SCV 56 is fully opened until the engine speed exceeds N4. The swirlratio may be set to, for example, about 1.5 to 3 within the SPCCIλ>1range and the SPCCIλ=1 range. Here, the opening of the SCV 56 may beabout 25 to 40%. Since the swirl flow is formed in the combustionchamber 17, the SI combustion in the SPCCI combustion becomes sharp,therefore, the SI ratio becomes higher than when the swirl flow is notformed.

FIG. 18 illustrates a relationship between the engine speed and thesecond-stage injection ratio within the SPCCI range of the operatingrange map 702 in FIG. 7C (waveform 113). In the upper chart of FIG. 18,the two-dotted chain line indicates the waveform 111 of FIG. 11. Asdescribed above, since the SI ratio becomes relatively high due to theswirl flow formed in the combustion chamber 17, the second-stageinjection ratio is set relatively low. When the second-stage injectionratio is low, since the amount of fuel injected in the first-stageinjection increases, a long vaporization time of the fuel is secured.This reduces the generation of unburned mixture gas and soot, which isadvantageous in improving the exhaust emission performance of the engine1. Note that also on the operating range map 702, the relationshipbetween the engine speed and the SI ratio conforms to the relationshipillustrated in FIG. 14.

Also in the waveform 113, as described above, when the engine speedexceeds N3, the second-stage injection ratio is increased to increasethe SI ratio of the SPCCI combustion. The second-stage injection ratioincreases as the engine speed increases. Note that the second-stageinjection ratio may be increased from an engine speed higher than N3. Inthe example of the waveform 113, the second-stage injection ratio iscontinuously increased at a given change rate as the engine speedincreases. Alternatively, the second-stage injection ratio may beincreased in a stepwise fashion (i.e., discontinuously) as the enginespeed increases. Since the SI ratio in the SPCCI combustion increasesand the combustion noise caused by the SPCCI combustion is reduced, whenthe engine speed is high, NVH is suppressed below the allowable value.

While the second-stage injection ratio is increased as the engine speedincreases, since the initial second-stage injection ratio is low, thesecond-stage injection ratio does not exceed the upper limit value evenat the speed N2. That is, by forming the swirl flow in the combustionchamber 17, also within the highest engine speed range of the operatingrange in which the SPCCI combustion is performed (the range near thespeed N2 in FIG. 18), when the engine speed is high, the second-stageinjection ratio is increased to be higher than when the engine speed islow.

Note that although not illustrated, when the engine speed is equal to orless than the given speed N3, the change rate of the second-stageinjection amount may be increased as the engine speed increases, insteadof setting the change rate to zero. In this case, the change rate whenthe engine speed is equal to or less than the given speed N3 ispreferably lower than the change rate when the engine speed exceeds thegiven speed N3.

Further, as illustrated by the one-dotted chain line in the upper chartof FIG. 18, the speed N3 at which the increase of the second-stageinjection ratio starts may be shifted to the higher engine speed sidewhen the engine load increases. In this manner, the range where the lowsecond-stage injection ratio is maintained is extended to the higherspeed side. If the second-stage injection ratio is low, the CIcombustion in the SPCCI combustion increases, which is advantageous inimproving the fuel efficiency.

Note that different from the example of the upper chart of FIG. 18, asindicated by the one-dotted chain line in a waveform 114 of the lowerchart of FIG. 18, the slope of the straight line indicating therelationship between the engine speed and the second-stage injectionratio may be set gentler at a high engine load than at a low engineload.

FIG. 19 illustrates a relationship between the engine speed and thesecond-stage injection timing within the SPCCI range of the operatingrange map 702 in FIG. 7C. A waveform 123 in the upper chart of FIG. 19corresponds to the waveform 121 in the upper chart of FIG. 12. Thetwo-dotted chain line in FIG. 19 illustrates a part of the waveform 121in FIG. 12.

Also in the waveform 123, when the engine speed exceeds the given speedN3, the ECU 10 advances the injection timing of the second-stageinjection as the engine speed increases. When the engine speed is high,the combustion noise of the SPCCI combustion is suppressed low. Notethat the injection timing of the second-stage injection may be advancedfrom an engine speed higher than N3.

Although not illustrated, when the engine speed is equal to or lowerthan the given speed N3, instead of setting the injection timing of thesecond-stage injection to zero, it may be increased as the engine speedincreases. In this case, the change rate when the engine speed is equalto or lower than the given speed N3 may be lower than that when theengine speed exceeds the given speed N3.

If the swirl flow is not formed in the combustion chamber 17, when theinjection timing of the second-stage injection is advanced, the flow inthe combustion chamber 17 caused by the injection becomes weaker at theignition timing. On the other hand, if the swirl flow is formed in thecombustion chamber 17, even when the injection timing of thesecond-stage injection is advanced, the flow in the combustion chamber17 caused by the injection at the ignition timing is kept strong. Thatis, by forming the swirl flow in the combustion chamber 17, the advancelimit of the second-stage injection is eliminated, and as illustrated bythe waveform 123, the injection timing of the second-stage injection isadvanced as the engine speed increases. By forming the swirl flow in thecombustion chamber 17, also within the highest engine speed range of theoperating range in which the SPCCI combustion is performed (the rangenear the speed N2 in FIG. 18), the timing of the second-stage injectionis advanced at a high engine speed than at a low engine speed.

When the second-stage injection timing is advanced, a long vaporizationtime length of the fuel is accordingly secured, and thus, the generationof unburned mixture gas and soot is reduced. Particularly, since theinjection amount of the second-stage injection increases as the enginespeed increases as illustrated in FIG. 18, advancing the second-stageinjection timing is advantageous in securing the long time length forthe fuel vaporization by the second-stage injection. This reduces thegeneration of unburned mixture gas and soot, which improves the exhaustemission performance of the engine.

Further, as illustrated by the one-dotted chain line in the upper chartof FIG. 18, the speed N3 at which the second-stage injection timing isstarted to be advanced may be shifted to the higher engine speed sidewhen the engine load increases. In this manner, the range where theinjection timing of the second-stage injection is late is extended tothe higher speed side.

Note that different from the example of the upper chart of FIG. 19, asindicated by the one-dotted chain line in the waveform 124 of the lowerchart of FIG. 19, the slope of the straight line indicating therelationship between the engine speed and the injection timing of thesecond-stage injection may be set gentler at a high engine load than ata low engine load.

(Example of Combustion Waveform of SPCCI Combustion)

FIGS. 20 and 21 illustrate combustion waveforms in operating states W1to W12 of the engine 1. An operating range map 704 illustrated in FIG.20 conforms to the operating range map 702 in FIG. 7C. That is, theSPCCI combustion is performed within the low load range including theidle operation and extending in the low and medium speed ranges on theoperating range map 704. This range corresponds to the low load range(1)-1 on the operating range map 702. Note that this range on theoperating range map 704 of FIG. 20 is a range in which the boosting isnot performed.

Further, on the operating range map 704 of FIG. 20, the SPCCI combustionis performed within the medium load range where the engine load ishigher than the low load range and within the high load range where theengine load is further higher. This range corresponds to the medium loadrange (1)-2 and the high-load medium-speed range (2) on the operatingrange map 702. This range on the operating range map 704 of FIG. 20 is arange in which the boosting is performed.

Further, on the operating range map of FIG. 20, a low speed side of thehigh load range where the engine speed is lower than the medium speedrange is the range in which the retarded SI combustion is performed.This range corresponds to the high-load low-speed range (3) on theoperating range map 702.

Additionally, on the operating range map 704 of FIG. 20, the high speedrange is the range in which the fuel is injected on the intake stroke toperform the SI combustion. This range corresponds to the high speedrange (4) on the operating range map 702.

Also on the operating range map of FIG. 20, as illustrated in FIGS. 11and 12 or FIGS. 18 and 19, within the range where the SPCCI combustionis performed, the second-stage injection ratio and/or the injectiontiming of the second-stage injection are changed according to the enginespeed changes. The engine speeds N1 and N2 of FIGS. 11 and 12 or FIGS.18 and 19 correspond to the engine speeds N1 and N2 of FIG. 14. Withinthe range in which the SPCCI combustion is performed, the SI ratiolinearly increases as the engine speed increases (see FIG. 14). Sincethe combustion noise of the SPCCI combustion is suppressed low byincreasing the SI ratio, when the engine speed is high, NVH issuppressed below the allowable value.

As described above, within the SPCCI range, the SI ratio is increased asthe engine speed increases. When the waveforms of W2, W7, and W10 arecompared, the peak of the SI combustion gradually increases in the orderof W2, W7, and W10. As a result, the peak of the CI combustion graduallydecreases in the order of W2, W7, and W10. Thus, the generation ofcombustion noise is reduced as the engine speed increases. Note thatwithin the SPCCI range, the similar tendency is shown in the waveformsW3, W8, and W11 and the waveforms W4, W9, and W12, which are for lowerengine loads than W2, W7, and W10.

Other Embodiments

Note that the control of the engine 1 performed by the ECU 10 is notlimited to be based on the combustion model described above.

Further, the art disclosed here is not limited to be applied to theengine 1 having the above configuration. The configuration of the engine1 may adopt various configurations.

It should be understood that the embodiments herein are illustrative andnot restrictive, since the scope of the invention is defined by theappended claims rather than by the description preceding them, and allchanges that fall within metes and bounds of the claims, or equivalenceof such metes and bounds thereof, are therefore intended to be embracedby the claims.

DESCRIPTION OF REFERENCE CHARACTERS

-   -   1 Engine    -   10 ECU (Controller)    -   17 Combustion Chamber    -   171 Squish Area    -   25 Spark Plug    -   3 Piston    -   31 Cavity    -   56 SCV (Intake Flow Control Device, Swirl Generating Part)    -   6 Injector    -   SW6 Pressure Sensor

What is claimed is:
 1. A control system of a compression-ignition engine, comprising: an engine configured to cause combustion of a mixture gas inside a combustion chamber; an injector attached to the engine and configured to inject fuel into the combustion chamber; a spark plug disposed to be oriented into the combustion chamber and configured to ignite the mixture gas inside the combustion chamber; and a controller operatively connected to the injector and the spark plug, and configured to operate the engine by outputting a control signal to the injector and the spark plug, respectively, wherein after the spark plug ignites the mixture gas to start combustion, unburned mixture gas combusts by self-ignition, the controller outputs the control signal to the injector to perform a first-stage injection of the fuel and, after the first-stage injection, a second-stage injection in which the fuel is injected to at least form the mixture gas around the spark plug, and the controller also outputs the control signal to the injector to control a ratio of the injection amount of the second-stage injection with respect to the injection amount of the first-stage injection to be higher at a high engine speed than at a low engine speed.
 2. The control system of claim 1, wherein the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection changes at a given change rate as the engine speed changes, and the controller causes the change rate to be higher at a high engine speed than at a low engine speed.
 3. The control system of claim 2, wherein when the engine speed is equal to or lower than a first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection is constant even when the engine speed changes, and when the engine speed exceeds the first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection increases as the engine speed increases.
 4. The control system of claim 3, wherein when the engine speed exceeds a second given speed that is higher than the first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection becomes a given value.
 5. The control system of claim 4, further comprising an intake flow control device attached to the engine that is configured to adjust a flow of intake air introduced into the combustion chamber, wherein, when the engine speed exceeds the second given speed, the controller outputs the control signal to the intake flow control device to strengthen the flow of the intake air.
 6. The control system of claim 1, wherein the controller outputs the control signal to the injector so that an injection timing of the second-stage injection is advanced when the engine operates at a high speed compared to when the engine operates at a low speed.
 7. The control system of claim 1, wherein the engine includes a piston constituting the combustion chamber, the piston being formed with a cavity facing the injector, by indenting an upper surface of the piston, and in the first-stage injection, the fuel is injected into a squish area outside the cavity on compression stroke, and in the second-stage injection, the fuel is injected into the cavity.
 8. The control system of claim 1, wherein the controller sets a self-ignition (SI) ratio to be lower than 100% and sets the SI ratio to be higher at a high engine speed than at a low engine speed, the SI ratio being an index relating to a ratio of a heat amount generated when the ignited mixture gas combusts by flame propagation with respect to a total heat amount generated when the mixture gas inside the combustion chamber combusts.
 9. The control system of claim 1, further comprising a swirl generating part configured to generate a swirl flow inside the combustion chamber, wherein the controller outputs the control signal to the swirl generating part to generate the swirl flow inside the combustion chamber regardless of the engine speed, and wherein, at least within a highest speed segment of an operating range of the engine in which the spark plug ignites the mixture gas to start the combustion and then the unburned mixture gas combusts by self-ignition, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection becomes higher at a high engine speed than at a low engine speed.
 10. The control system of claim 1, wherein the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection changes at a given change rate as the engine speed changes, the controller causes the change rate to be higher at a high engine speed than at a low engine speed, when the engine speed is equal to or lower than a first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection is constant even when the engine speed changes, when the engine speed exceeds the first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection increases as the engine speed increases, and when the engine speed exceeds a second given speed that is higher than the first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection becomes a given value.
 11. A control system of a compression-ignition engine, comprising: an engine configured to cause combustion of mixture gas inside a combustion chamber; an injector attached to the engine and configured to inject fuel into the combustion chamber; a spark plug disposed adjacent to the injector and configured to ignite the mixture gas inside the combustion chamber; and a controller operatively connected to the injector and the spark plug, and configured to operate the engine by outputting a control signal to the injector and the spark plug, respectively, wherein after the spark plug ignites the mixture gas to start combustion, unburned mixture gas combusts by self-ignition, the controller outputs the control signal to the injector to perform a first-stage injection of the fuel in a period from intake stroke to an early half of compression stroke, and a second-stage injection in a period from a latter half of the compression stroke to an early half of expansion stroke, and the controller also outputs the control signal to the injector to control a ratio of the injection amount of the second-stage injection with respect to the injection amount of the first-stage injection to be higher at a high engine speed than at a low engine speed.
 12. The control system of claim 11, wherein the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection changes at a given change rate as the engine speed changes, and the controller causes the change rate to be higher at a high engine speed than at a low engine speed.
 13. The control system of claim 12, wherein when the engine speed is equal to or lower than a first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection is constant even when the engine speed changes, and when the engine speed exceeds the first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection increases as the engine speed increases.
 14. The control system of claim 13, wherein when the engine speed exceeds a second given speed that is higher than the first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection becomes a given value.
 15. The control system of claim 14, further comprising an intake flow control device attached to the engine that is configured to adjust a flow of intake air introduced into the combustion chamber, wherein, when the engine speed exceeds the second given speed, the controller outputs the control signal to the intake flow control device to strengthen the flow of the intake air.
 16. The control system of claim 11, wherein the controller outputs the control signal to the injector so that an injection timing of the second-stage injection is advanced when the engine operates at a high speed compared to when the engine operates at a low speed.
 17. The control system of claim 11, wherein the engine includes a piston constituting the combustion chamber, the piston being formed with a cavity facing the injector, by indenting an upper surface of the piston, and in the first-stage injection, the fuel is injected into a squish area outside the cavity on compression stroke, and in the second-stage injection, the fuel is injected into the cavity.
 18. The control system of claim 11, wherein the controller sets a self-ignition (SI) ratio to be lower than 100% and sets the SI ratio to be higher at a high engine speed than at a low engine speed, the SI ratio being an index relating to a ratio of a heat amount generated when the ignited mixture gas combusts by flame propagation with respect to a total heat amount generated when the mixture gas inside the combustion chamber combusts.
 19. The control system of claim 11, further comprising a swirl generating part configured to generate a swirl flow inside the combustion chamber, wherein the controller outputs the control signal to the swirl generating part to generate the swirl flow inside the combustion chamber regardless of the engine speed, and wherein, at least within a highest speed segment of an operating range of the engine in which the spark plug ignites the mixture gas to start the combustion and then the unburned mixture gas combusts by self-ignition, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection becomes higher at a high engine speed than at a low engine speed.
 20. The control system of claim 11, wherein the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection changes at a given change rate as the engine speed changes, the controller causes the change rate to be higher at a high engine speed than at a low engine speed, when the engine speed is equal to or lower than a first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection is constant even when the engine speed changes, when the engine speed exceeds the first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection increases as the engine speed increases, and wherein when the engine speed exceeds a second given speed that is higher than the first given speed, the controller outputs the control signal to the injector so that the ratio of the injection amount of the second-stage injection becomes a given value. 